If you put say, 800 watt amp to two 10" 750 watt rms subs would they perform better than two subs that have 400 watts rms or would they perform the same because its the same amount of wattage. or would the 750 watt rms subs have better sound qaulity than the 400 watt rms subs when putting an 800 watt amp to them.
It will have a lot to do with what type of box it's in and what the system is going in car or truck. I think the two subs wired at 2 ohms with 400 rms each will perform better than the 750 rms. I have not heard that sub on that amp so I can't say how the sound quality will be. I think the MTX 801d will pump on one dual 4-ohm sub. Let me know how it sounds. I'm thinking about getting that amp.
general rule of thumb. going from one sub to two subs = +3dB gain double the power (400 to 800w) = +3dB gain
so, one sub with 800 watts or two with 400 watts each should be about the same output. realistic results will vary, but usually you'd get better results using two subs with 400 watts each just because the above guidelines don't really account for efficiency curves or things of that nature, and many other factors will apply to the end result. The above also assumes that all other factors are totally equal, and they never will be.
it's going to boil down to which amps and which subs you're considering, what type of car and box, your listening habits and music tastes, etc.