Cabinet resonance problem with Marantz MM9340 amplifier

More info - I played the same music through the Home Theater system.   It isn't the same quality of components as the Big Stereo.  The deep bass was there but the muddy sound of the bass was not.   Given this, I am assuming the problem is definitely in the Marantz amp.

I believe I discovered about 90% of the problem.   There is a 1KVA, medical grade, toroidal isolation transformer that isolates my system from the grid.   Thus any line leakage to neutrals or grounds is blocked, eliminating any hum induced from leakage currents into the signal grounds.   Apparently, there is some sort of oscillation problem between the Class D Marantz and the isolation transformer.  Clearly, it is triggered by the music load put on the power supply which draws power from the isolation transformer.  It is important to note the Marantz also has a toroidal transformer in its power supply as well.  The two 200W monoblock Conrad Johnson Premier Fives, which are tubes, are not affected in any way.   Neither is the solid state Parasound 2250 V2 amplifier at 275W per channel. The Parasound has a toroidal while the Premier Fives do not.   But the Marantz, at 275W per channel, simply goes nuts!  About 90% of the resonance problem disappeared when I plugged the Marantz directly into the wall.  

I say 90% since there is still an unacceptable amount of vibration in the Marantz chassis.   I ordered 4 square feet of DynaMat Extreme sound deadening material, so a thank-you to all who suggested it.  I am pretty certain this will eliminate most of that 10%.   It is certainly thick and heavy enough!

As for the 1KVA grid isolation toroidal, the longer-term plan is to add two 7.5 KVA isolation transformers, along with the existing 1KVA unit, and run them all from 240VAC, thus generating three independent separate derived supplies for the stereo.  It appears I need to move that time frame up!.   If I still have a problem, then it might be time to look at a new amplifier.  I am sure there are amplifiers with better sound quality out there that will drive subwoofers quite well.

Although I am on a dedicated circuit, I own a particular Hafler designed amp ( Smart Theater Systems TA 242 I acquired from a going out of business movie theater ) that uses a toroidal transformer. When connected through my system, the transformer " hums " when I am using my OTR microwave. From the chassis only, not through the speakers. Yes, I damped this chassis and isolated the transformer using a few layers of Peel and Seal, and the hum is much lower, but I can still hear it. I suppose I can stop the humming by teaching it the words.......not my joke....Enjoy !

@mrdecibel That hum is caused by a DC offset in the power line.   It happens and only a volt or three will cause it.   There are several devices made to eliminate this, as the problem is worse with toroidal transformers.  If you Google "Blocking Mains DC Offset From Transformers", you should find lots of stuff to fix that.   There is a good article here: https://sound-au.com/articles/xfmr-dc.htm

If you scroll down a bit on that article, you will find a schematic so you can home brew it.   I did cost it out and found you can buy ready made DC Blockers for about the home brew price, assuming you bought parts from DigiKey or Newark Electronics. 

Which, BTW, reminds me I should try a DC Blocker to fix my problem!

Some article. TY. I guess it would not harm anything by investing in one.