My take on using an external amp from an AVR...
Lower Price, light weight, high power... pick two.
I use one for my LR. I don't care as much about SOTA for surround channels, so long as I don't hear obvious clipping, or some such while playing movies at reference volume, and I don't with my Denon X4000. While most amps today tend to sound more alike than different, there are some things I appreciate in a separate power amp which do provide superior audio performance.
First is better heat management, I love the AVR I have but I can slow roast a brisket on the thing. Fortunately I can turn off the front channels, and that helps a little so the AVR can just focus on surround. Another issue is power supply. Except for the absolute best AVRs (the one's more than 2k, and sometimes not even those) the manufacturer can't make money with a proper power supply, and fit everything else in a chassis that needs to be a certain size or that many more consumers won't buy it. Many AVRs simply can't handle much below 8 ohms, 6 ohms minimum, to say nothing about adequate <4 ohm performance.
That said, I got an old Denon POA 2800 power amp that I got used from a friend for $50. Price matters to an extent, but it doesn't take much, especially on the used market. Just be careful with used amps, an amp can last 20 years, blow it's lid, and there's no shame; it still had a good run. But, the next one on the line could last decades longer. So if you buy an old amp, tuck this little tidbit away.
New video from Gene