[Question #1] If I'm using the 791 7.1 channel receiver for a 5.1 system, am i wasting power or will it use the unused 2 channel's power for something thats plugged in?
Typically receivers use a shared power supply which means that if you aren't using all 7 channels that are amplified, the power supply can use the excess power for the speakers which are connected and because there isn't as much load on the power supply it will help the receiver last longer.
Generally, there is no such thing as 'to much' power. If you don't use what you have, it may be wasted power, but having extra power is generally a good thing. Still, audio in not linear in terms of power to audio level and speakers respond differently, so the difference between a 20 watt amplifier, and a 200 watt amplifier may be a lot less than you may believe.
[Question #2] Is this because its not powerful enough or because of ventilation? It's sitting on carpet at the moment with other compenents stacked on the sides with nothing on top of it. 85% is plenty deafining but quite often I get in the mood to try to damage my ears and there cannot be too loud as long as its clear.
When you work an A/V receiver, it needs plenty of ventilation. So, get it off the carpet a bit, and pull stuff away from the sides. Still, when driven hard, if may go into a safety shutdown just for protection. A larger, more powerful amplifier won't have this issue.
[Question #3] How do you pair a receiver to your speakers when there is a big difference in the max power each speaker can take like in my main theater setup?
Speakers should be matched to handle incoming audio loads. You can adjust power levels of the other speakers downward, but you really can't adjust the peak level downwards. Bottom line is that if you are trying to drive the speakers to hard, you will break the speakers more often than the receiver driving them.
Larger, better built speakers, will handle more power. Smaller speakers just won't do it.
[Question #4] What wattage receiver would be ideal for my main theatre setup?
An amplifier of this caliber wouldn't be inappropriate...
Emotiva Audio XPA-5 Five Channel Power Amplifier
[Question #5] What is the ideal wattage receiver for my Pioneer SP setup?
About any Denon A/V receiver would be fine.
[Question #6] What is the ideal wattage receiver for my Quintet setup?
Same as number 5 - any should do fine.
If you have a need for loud audio, then you will want to focus on quality speakers, and then discrete amplification which provides sufficient power to those speakers. Smaller speakers, like the Quintets, will sound good at lower volumes, but they will NOT deliver as volume is turned up. They simply can't do it - it's not what they were designed for.
Even getting a higher quality sound from a speaker doesn't mean that it is designed to play loud. Generally more drivers and larger size help to create more sound, and there are certainly speakers out there which can deliver if that's your goal.
