When AVR's are powering multiple speakers, the amount of power per speaker usually decreases. So having an additional external power amp in the mix, can be helpful.
I have used the Beresford speaker switch, and it is excellent, but it requires a surprisingly lot of cabling and banana plugs, and it is a manual switch. A stereo amp with HT bypass is easier.
But even easier still...
I thought you already had the AVR. If not, let me point you to a very rare bargain in these times:
The AVR390 offers a level of sound quality that is far above competing receivers, these AVRs are audiophile products by any measure. Equally at home with high resolution surround sound or two-channel music, the AVR390 delivers stunning realism with everything from heavyweight blockbusters to...
petertyson.co.uk
It is an end of line FMJ model, but the replacement AVR10 is twice the price. It is missing eARC and HDR10+ passthrough, but there are workarounds. The main thing is that it sounds excellent for both movies and music.