You are conflating PCM and DSD which are entirely different things. “PCM up to DSD128” is not informative because it means nothing.
A one-box solution – (since these are Rose forums) is RS-520 and a speaker-level switchbox.
Alternative solution, since you must have some power amps with that AVM90, either RS-150 or RS-250 and a line-level switchbox to the power amplifier for front channels.
Easiest though, would be either RS-150 or RS-250 (really, depends only on the size of screen you want) connected to the AVM90 with HDMI. That way you can use ARC (which is a good thing).
I know that it is a very goof HT processor, it’s on my shortlist for when I finally decide to use the “media room” as a media room instead of a warehouse The problem is that it is very much designed for home theater work and does not make much concessions to straight stereo. Other than using the HDMI connection there is no way to get (very) high resolution signal in (well, strictly speaking 192/24 that you could get in over S/PDIF coax would not sound any different from 768/32, but I understand how annoying it could be to know that it is available, but you can’t get it), nd there is no direct analog input, anything analog would be digitized to 192/24, so using a “better” (or higher resolution anyway) DAC is pointless.
If you didn’t have it yet I would suggest something like Marantz AV10 which has a pure direct analog path, or a Primare SP25 which has a proper USB input and can handle 768/32 PCM or DSD512 (both support Dirac Live, which shouldn’t be any worse than ARC). But since you already have the AVM90, just use HDMI from 15/250.