Just to note--your avr amp would be running center and surrounds only--the powered subwoofer has its own amp.
I have a tube amp with quality front LR speakers for 2-channel listening, and use the AVR for rear surrounds.
You don't mention that you have purchased center or surround speakers yet. I think you might find a center speaker, powered by the AVR amp, to sound somewhat different from the tube-powered LR horns. The speakers are the main thing--if you get an identical speaker in the center you might be OK. But otherwise you might lose the smooth front soundstage that you want for movies. I chose to leave out the center (phantom center) and got better results in my system. With a good stereo amp and front speakers you should have a very smooth soundstage up front and not need a center IMO. Generally I think the farther off to the side you sit (the greater the viewing angle is off the perpendicular, the more you would notice the lack of a real center.
If you skip buying a center you can spend more on better surrounds, that more closely match your current speakers. However, again IMO, for movies the matching of surrounds to fronts is not as key as getting center and front LR to match. For multi-channel music it's a whole different thing--I won't discuss that here.
My AVR only powers the two rear surrounds currently. It's useful for that, although I am considering an outboard amp for those as well.
Just make sure you get an AVR with preamp-outs. The less-expensive ones often don't have those.