There ain’t nuthin’ wrong with the amplifier section of that Sony unit.
It is well built and sounds good.
That’s a 24.5kg (55lb) unit and quite a bit of that weight is the power amplifier.
Particular attention was paid in the ES units. Eg, even the heatsink was designed to help the amplifier sound good.
I see that you are up to the idea of making parts changes, and that changes the equation again, to your favor. Some better coupling caps and maybe a few cap changes to the sony amp section, would really make it shine. Then you’d be in a position where only very close attention to detail (in searching) might -I mean might- make for a better amplifier choice....but only at the upper end of the market at the 3-5kilo buck range.
At such a point, if you don’t require the extra power or headroom and are not stressing things too much, then the Sony, as a power amp... when economically self modified/upgraded, will be the clear financial and quality (paired considerations) winner.
I’ve done this sort of thing a literal hundreds of times. The answer is always the same.
Also look at the preamp output section of the Rotel, as it is the source point. A few mods to the output section will pay major dividends. Better coupling caps for the output section lines (they will be capacitor coupled) and recapping the power supplies that are local to each op amp that is the output of the processing section..that all pays off at least as well as mods in the Sony amp.
The tech manuals for both are available, IIRC, so this is more like the tech’s version of shoveling snow or mowing the lawn, it’s standard easy audio tech fare.
