Weak link, what do ya think ?

Normally I would have to say its the wire. Or in this case lack thereof. No name wire is a huge red flag. In this case though you have an AV processor. There are no good ones, so that would have to be the weak link. 

I consider Monster to be a wholly unethical company, so I would jettison the HTS.

I made the mistake of buying a Monster Power Conditioner $500+. It actually ceased to function and the company said tough… it cannot be repaired. No independent shop would cover it. I had it for my HT… I would never use one for my audio system. 

Since you’re posting this here I’ll assume 2-channel is important to you.  I’d start by unplugging everything from the Monster (because it’s quick and free) and then buy a good stereo preamp and run the front L/R preouts from the Marantz into the stereo preamp.  I think both those changes would make your nice Grand Pianos (and you) much happier.  Best of luck.