Adding 5-channel amps to AV Receivers

I'm currently shopping for a used 5-channel amp to use with my Denon x3400h receiver with a 5.1.2 Atmos setup, speakers are the older SVS SCS1/SB1 which still do great.  This setup is 100% movies/video games, music is for the 2-channel setup upstairs (Levinson, Revel, PS Audio).  The home theater is never going to get the budget that the music system gets, but its still a lot of fun and Dolby Atmos is really cool with down-firing ceiling mounted speakers.

My question - It seems like adding a separate 5-channel amp to pretty much any AVR would be a very standard setup for anything above a low-end home theater setup, but I rarely see it discussed.  IMO its asking a lot of a sub-$1k receiver to handle all the processing and 7 channels of amplification with its single power supply.  When you can buy a used Rotel, Parasound, etc 5-channel amp for less than $500 and let the AVR be the processor, this should be a no-brainer right?  Pulling out at least 5 channels to a real amp should have all kinds of obvious benefits.  Even if the speakers are not full-size, reasonably efficient, and there's no clear need for more power, this should still produce much better sound.

It seems like an obvious move but its hard to find any discussion of it, usually the conversation goes from budget AVR to high-end AVR to separates.  Am I missing something here?