Suggested upgrade paths

Please stay away from changing cables to correct this problem, too subtle and subjective. 

The most cost-effective way, as stated above, is speaker placement. Try toeing in, or out

That's a classic symptom of too live of a room. Whatever you can do to absorb some higher frequencies would help. Carpet, drapes, artistic acoustic panels, etc. are what will help most. 

Not being familiar with your gear, I would start by reading reviews on my gear to see if the issue with sibilance or bass is common to any of my gear.

I would guess that the bass issue is speaker related.  Either placement, the speaker itself, or both. 

Sibilance drove me nuts for years. I tried different cables which kinda helped by rolling off the top end a bit, but it didn't solve my problem.  I just blamed it on poor recordings. Thinking my gear is now so revealing that all of the crap recordings are more noticeable. 

My problem turned out to be a well regarded tube preamp.  Replacing it pretty much fixed the problem.  Upgrading my dac also improved things. Now, I have a revealing system and I don't even think of sibilance. At one point, I had almost concluded that sibilance was just something that I'd have to live with.  I was wrong. 

since your complaint is about the sound quality when it’s louder, I would upgrade the amplifier to a seperate. You could still use the rotel as a preamp.

You’re getting distortion, and I assume “65” is out of 100, which sounds like you operate at max gas. So far its been my experience that amps distort before speakers do, assuming decent speakers which you have.

Next would be a speaker upgrade if the amp doesn’t help. Try a few.

I owned the Lintons. Both the Super Linton and the OG are terrific speakers, but they need careful setup to really shine. The A6 and the Rotel’s internal DAC are the weak links. I recommend replacing them with a good streamer and an R2R DAC, since you don’t like delta-sigma designs.

The Node ICON ($1,200 new) with the Teddy Pardo LPS mod ($485) is a highly regarded streamer option, and the Bluesound app runs smoothly. The ICON is even being praised by TAS as an easy alternative to a $35k Aurender N200 and a $28k DAC combo, though I think pairing it with an external R2R DAC would do it more justice.

A Harmony DAC ($2,700) with the Micro DDC ($850) would make a terrific combination. Altogether, the upgrade would run you just over $5k.