When upgrading speakers, several things matter.
Three standouts:
1. The quality of the drivers (speakers).
2. The quality, size matching, and overall size of the "cabinet."
3. Efficiency
Buying decent drivers is pretty easy, but getting decent "surface-mount" cabinets for something as big as a 6" or 6 1/2" coax speaker is a real challenge.
Speakers are designed with an anticipated cabinet size in mind. A 6" coax meant for car-door installation will ideally be different than a 6" coax designed for rear deck (auto) installation, because the "cabinet" space is different. Many super high quality rear-deck speakers actually have cabinets that hang on the underside of a rear deck to isolate the speaker from the enormous volume of the trunk and properly load the woofer. Other rear deck speakers do not rely on "cabinet loading" of the rear of the woofer to contain its excursion like an air spring. The speaker surround handles that...at some cost to fidelity. And, as most of you know, those big thumping subwoofers typically have enormous cabinets that take up much of the trunk space. Size matters.
This brings us to the type of surface-mount speaker enclosure used in boats and to hang on the outside of an RV. Marine speaker systems are designed as a matched set...the cabinet and drivers are designed as a unit. The mounting surface often forms the rear of the "cabinet", but better quality surface mounts have their own back on their cabinet...leaving nothing to chance. The cabinet is far too tiny under "normal" circumstances, and that tiny speaker cabinet restricts the excursion of the woofer. But it is possible to design a matched set...woofer and cabinet...to perform as well as possible under these challenging circumstances...and get the most out of a difficult application.
The quality of that enclosure matters, too. Simple ABS plastic boxes are not rigid or dense enough to cope with the vibrations, so they add their own vibrations that deteriorate the sound. Good cabinets are dense and rigid...and expensive.
Consider your home theater speakers. Even modest speakers that have a 6" woofer will have a cabinet that is, perhaps, 12" tall x 7" wide and 6" deep (interior volume). That's roughly 500 cubic inches of interior volume. Even a sound bar has a surprisingly large interior volume. Meanwhile, a marine speaker for a 6" coax might have an interior volume of only half that or less. Your home speaker might also be ported (to allow sound pressure from the back side of the woofer to come out of the cabinet, while the marine speaker is not. So the marine speaker cabinet severely limits woofer excursion all else being equal.
Fortunately, all else does not need to be equal. The specs of the 6" woofer in the coax speaker can be adjusted to allow greater excursion (the energy that makes bass) in the smaller cabinet.
So it makes sense to buy purpose-built marine speakers rather than pick cabinet (plastic enclosure) and speakers separately...with one caveat. The act of choosing an "outdoor" speaker" typically leads to choosing a speaker that is likely to be a good match for a marine enclosure. But not always. It's also possible to buy weather-proof speakers that can be mounted in the walls of, for example, a home, and the enclosure would be the wall cavity behind the speaker...a very large speaker cabinet. And then there are marine applications where the speaker is installed in a "cabinet" much like a car door enclosure (in the "dash" or in the side of the cockpit)...or with no backing whatsoever. Matching the driver to the application is important.
Unless you are a sound engineer, if you want to upgrade outdoor, surface-mount speakers, I recommend buying the best quality, purpose-built system you can afford.
Polarity matters. While it's OK that interior speakers and exterior speakers are out of phase, that's only the case if the doors and windows are closed. All speakers should be in phase. Why? Because at 40 hz, about the lowest bass you're likely to hear, the sound wave is
28 feet long!
The WORST case scenario is a speaker pair (inside or outside) out of phase, in which case, the bass cancels itself out. But if the inside and outside speakers are out of phase, there will still be
erratic phase cancellation that will severely undermine bass output....up to the low midrange.
It's reasonable to assume that the "stereo" (sound system) head unit is built with all channels in phase. It's also reasonable to assume that the RV builder used phase-coded wire (one side of the speaker wire pair can be identified from the other), so if you have any doubts, pull the head unit and make sure that all speaker wire is in phase (the coded side of the wire always goes to the + or to the - terminal on the head unit). If that's correct, the RV is wired correctly. Then check the speaker connections and be sure all are connected the same. If so, you're in phase. If not, make the correction at the offending speaker. If you correct an out-of-phase situation, you'll be stunned at how much better things sound.
Ironically, it does NOT matter if the "+" on the head unit is connected to the "+" speaker terminal. All that matters for proper polarity is that they are all the same. Let's assume a black and white wire (jacket) pair. Black can go to the + on the head unit and go to - on the speaker, but if they are all wired the same, everything's good. Ideally, it makes sense to go + to +, but that's just to save confusion later.
Finally, on efficiency. Really efficient speakers get loud with less amplifier power than less efficient speakers. This matters because the head unit on most RV sound systems typically makes only 5 watts a channel (RMS) regardless of what they say. It's a simple fact of life that, without inverting 12 volts to higher voltage (how they power those monster amps that drive the big thumping subs) you can't squeeze blood from a stone. Reasonable current at 12 volts limits most "car" stereos to 5 watts a channel unless you resort to tricks that deteriorate the sound. So, to get "louder" with such puny amps, the best answer is highly efficient speakers.
If you question the output of a typical "car" stereo,
run the numbers. If your head unit is fused at 5 amps, that means it NEVER draws 5 amps or the fuse blows. But using 5 amps and 12 volts and typical 4 ohm resistance per speaker channel, you have only 36 to 60 watts to divide among 4 channels, run the CD/DVD, power the face panel and do everything else going on in the head unit. If you want to argue that there's more power available, fine, I'll concede that you might have 7 or 8 watts, but then the total harmonic distortion starts climbing beyond 1%...which is more than audible.
Since big amps in cars are now "cheap and easy," be wary of great sounding speakers that are inefficient. The might sound great in "Car Toys" when running with 50 watts a side, but 5 watts will barely make any noise out of them. If you can find this rating, an efficient speaker will make 90 or more dB SPL (decibels sound pressure level) at 1 watt at 1 meter. dB is a logarithmic scale, so 87 dB is MUCH less than 90 dB. 93 to 96 dB SPL @1watt/meter is a very efficient speaker. But in something as prosaic as a marine speaker, it may be difficult to find that spec. (look on the manufacturer's site.) In this case, you may be stuck estimating efficiency by using the power handling specs of the speaker...as in this
Crutchfield list. Note that the top-selling Kenwood has a power handling recommendation of 2 watts to 50 watts, whereas many of the "better" speakers are recommending 30 watts or more as minimum power. Remember that the little Furrion in your RV probably only makes 5 "real" watts per channel. If you install some of those sweet JL Audio speakers, they'll be pretty darned quiet...and disappointing. And if you upgrade your amps, you won't be listening while boondocking unless you like your music with a side of generator.
Credentials:
a) 22 years in commercial broadcast TV. Last gig running all tech operations at an NYC indie news station that had 4 studios...one in Manhattan.
b) custom loudspeaker design, including crossover networks, cabinets, and driver selection from OEM manufacturers for major brands.
c) custom sound-reinforcement speakers for music festivals and for Arlo Guthrie when touring.
P.S. My HW PUP came through with some truly bizarre speaker wiring. The "balance" control adjusts relative volume inside and outside. The "fade" control adjusts "balance - left/right volume" inside (my PUP has a single speaker outside...which I presume is mono, but who knows?) This lunacy is hard to figure, and I'm convinced that this craziness has deprived me of stereo other than if I sit in the door jamb and listen to the inside and outside speakers simultaneously.
Since I boondock exclusively, and since I do it with a single 100 watt solar panel feeding a single group 24 battery, I NEVER use the built in sound system. If I want music, I use a "device" of one sort or another connected to a bluetooth speaker. A typical "car" stereo pulls close to 5 amps, and my battery has between 35 to 40 usable AH, so I'm not wasting those precious amp-hours on a crappy sound system.