Looking for power amp with warm, smooth sound

I don't know how it would pair up with your Marrantz, but you might consider a Pass Aleph 3 or 30.

Cayin A-50T - tubes and wonderful. $1,200 new should be doable.

I have one into B&W 685's and it sounds sweet. Very sweet.

The RS6's are a pretty bright speaker. I may consider trying to sell them and use the money you set aside for an amp towards a speaker upgrade.

Second the Cayin A-50T...excellent amp! Sonics, cosmetics, build - all first rate!!!

BAT VK200
ODYSSEY STRATOS SE
ROTEL 1080

Any of these should do it.

If not... and even before throwing another amp at things, maybe adding a power filter for a couple hundred might help, like the UPC 200 from PS Audio.

If the receiver has a migratory power cord, that option too should be explored.

I overlooked cabling and conditioning early on and went through a lot more $$$ than I might have, had I looked into those accessories as a solution. Instead I began replacing components and that's an expensive lesson as ultimately they needed to be addressed.

Good luck.