because, by definition, a 5.1 receiver has only six channels of amplification, not the eight channels needed for 7.1.
I alluded to a time when it was commonly available as feature to use the pre-amp section of the receiver (or amp) independently from the basic power amp section. One could run another, supplementary amp through this combination to make decent use of an otherwise perhaps obsolete or insufficient amp. So two power amp sections would be managed by the one pre-amp section.
At this relatively high level of concern, such as the Emotiva & Onkyo 805 & stepup to 875 or Sony 5300, they are all pretty powerful & frankly, more power than most typical users need. And, if these are within your budget, you probably can't go too far wrong with any of them.
Yeah, example that I use a reference grade receiver to in effect just waste some channels because I only use four of its power amp channels to bi-amp some pretty high grade speakers. So I am actually playing games of excess & not using that piece of equipment efficiently. But, hey, it's just my game to play.
You may still be a bit confused. No, the pre-amp function is not taking on the surround sound function while the basic amp section provides power. Yes the basic amp does the providing of power by means of however many channels it has. These include all the channels; ie. front L&R, center channel, sub woofer feed, side surrounds, & rear surrounds when present. The pre-amp is the control board for making all the adjustments.
To revert to the fundamental basics, many of us do like to emphasize that, because your personal touch for the system lies mostly all in which set of speakers you do ultimately select, that many are attracted by the advertising hub-bub about receivers when they should key in on comparing several good candidates to arrive at your own individual most desirable preference of what speakers suit you the best, & therefore, maximizing the speaker part of the budget, while otherwise sacrificing some of the budget for the power source because so many get more than essential levels of receiver power. The goal might be 75% for the speakers to 25% for the receiver.
It's obviously your choice & even if it involves some dedicated & unanticipated work to make your most appropriate choices. And, heck, the purpose is to get much enjoyment from the sound. So do some educational work & wind up enjoying.