There are test sources that will play all channels loud. Or pretty much any source if you shift your receiver into "7 Channel Stereo". _______
Go look at some of these receiver specs if you want. Mine says something like '110wpc, RMS, <.08%THD, Left + Right'. Then it has 3 more lines that basically duplicate this saying 'Center', 'Left Surround + Right Surround', and 'Left Back Surround + Right Back Surround'. The key point is it only says "Left + Right' on that first line, meaning it's only measured to 110wpc with those 2 channels driven. Same for the other lines for the other channels. Tricky language, no doubt. So, it's just a testing/marketing number, it isn't supposed to match to your real world playback of music or movies.
Now, a quality power amp will have better specs. The Sunfire Cinema 7 Signature, for example. It says 400wpc, RMS, All Channels Driven, <.5%THD. So, this amp should be able to actually put out 400 wpc if driven hard, and 3rd party tests usually show Sunfire amps to be underrated by a few wpc. Unfortunately, Bob always rates his with higher THD ratings than most (.5% is quite high), kind of annoying for comparisons.
But like I said above, you won't get that for $1000. The Sunfire is something like $4000, and it's just an amp, no preamp. Other good amps with honest 100-200wpc ratings are often in the $1500-2000 range. _______
For the issue of input power, that usually is not a problem. Today's receivers generally have circuits to counteract this issue. My Onkyo can be set with a +-10db adjustment to each input so the volume will be at similar levels if one source is much quieter/louder than another.
This was a big problem for people using unpowered SV subs with certain receivers that have a surprisingly low output voltage on the subwoofer preout. These people bought good pro amps, but ran into the exact problem you mentioned. That is, the sub, powered by the external amp (without gain), couldn't match the output of the other speakers. So they had to put a gain box in between the receiver and external amp. Not the slickest solution, but this would work with extreme cases going from source to receiver, also.
