I know it sounds crazy, but all I can tell you is NEVER UNDERESTIMATE THE NEED FOR POWER. The more juice you have, the better it will sound at a normal volume.
Think of amps like a reel. A small baitcaster will be able to fight a 20# striper, but it will have to work harder to do it. But if you had more reel, it would handle the fish much better, with less stress.
Well when it comes to amps, the hormonic distortion (what makes speakers sound bad and unbalanced when cranked), is mostly found on the top 30% of what the amp can handle. So, a 200 watt amp sounds better at 10% than a 20 watt amp at 100%. (In fact, a speaker can handle much more power than it's rated for, without disorting, as long as the signal from the amp is clean) Also, the bass frequencies require much more power to acheive a comfortable volume than higher frequencies. That's why guitar amps usualy max out at 100 watts, but bass amps can be found at 500+ watts.
Unfortionalty, power is everything. (well, almost everything). Manufacturers are still trying to convice us that they can "pack great sound into a small light box", but until they can alter the physics of sound, it's just not going to happen. A 50lb sub will ALWAYS sound more full than a 5lb sub.
That's why real hard core stereo geeks still have their 150lb JBLs in there mom's basement.... ;-)
Here are a couple of suggestions.
1) don't put too much money into the sides and rear. Put your efforts into the center channel instead. A lot of people put too much emphasis on the sides and end up with a un-natural sounding system. (phasing problems) Remember for surrond to sound good, it needs to be convicing. And unless you have a 20' screen, your brain is looking (and listening) the the focal point (center channel). Use the sides and real for fill only.
2) Buy a self powered sub. That way you can get enough power (without having to loose power on long cable runs), and you won't waste power from the main amp.
On whatever you buy, read the specs carefuly. Pay particular attention to the impedance of the signal (they boast "more power" by quoting a lower impeadence) and the signal to noise ratio.
Now just get a TiVo for the fishing shows and you are rolling!!!!