Very interesting post. Im glad to see your very concerned about your stereo and how it sounds, most people are not
Your bring up some good arguments.
The first thing I should mention though is how was it all installed. Remember a good setup is not just good components, but install is key as well. 70% install and 30% equiptment, or something around that ballpark.
You get what I mean though.
I woulnd't focus too much on buying high end compoents, but rather spend a little less and install it properly, get a good EQ/crossover and tune it well, it can make a night and day difference.
Plus all that pricey stuff in a boat enviroment that is very open, is kind of a waste IMO. You are limited in ways.
As for the sub debate, it can go on and on and on. People used to say 15" were sloppy and 10" were tight. Well it has more to do with the mechanical design of the sub and the type of enclosure its in than the actual size itself. I have heard some pretty weak 10" subs that sounded like crap and could not properly control the cone movement, and I have heard some very beefy 15" subs that could reproduce a bass transient with ease and make it sound inceredible.
How were your friends subs installed? Were they in the right size enclosure? what type of enclosure was it? How were the subs positioned? How much power was being fed to them. These are some factors that will greatly affect the overall sound.
Larger subs also have the ability to move more air which also helps in reproducing those low notes. Small subs have their strenghts, (I also run 8" in my car) but sometimes there physical markup hinder their performance in some areas.
Midbass drivers can really evenout a system. Sometimes midbass is really lacking and its annoying on your ears if you dont have decent mid bass. A good 3 way component setup with tweeter, 4" mid or 6.5" mid and 8" midbass can really mellow things out. There is always a gap between the sub crossover and the mids that can't always be reproduced by a small speaker like a 6.5" mid, so you have a gap that is apparent that the sub cannot accurately fill and neither can the 6.5", thats where a midbass can come in handy. You can try and play around with correct tuning to make the sub and mid overlap a bit but sometimes it just doesn't sound right.
Question 2: See above mentioned. There isn't just 1.
What is your optimal goal? Accurate reproduction of the music, or just an overall loudness in the boat with good overall sound (more feasable).
If you want to go with A, youd want to make up some kick pods/panels and properly aim your speakers for proper channel seperation and for soundstage, but in a boat its really tough and kind of stupid because your never in the same position with multiple positions to sit in.