Cabinet resonance problem with Marantz MM9340 amplifier

@fiesta75 - Well, I can call them, but it is out of warranty, so it would be at my expense.  At 80 lbs, freight & insurance won't be cheap.  The thing is, I am sure they will say "It meets specs, nothing wrong with it".  There is nothing in the specifications that the cabinet is resonance free.  And, it does work perfectly, other than vibrating.

@erik_squires - I can definitely feel it, the cabinet vibrates like crazy.  It is a steel chassis, so internal magnetics can be the cause.   That was my thinking about using a thick plexiglass cover.   

I've used spray on 3M rubberized undercoat, P/N 03584 on the inside of a CD player cover with success. Good luck.

Do you feel you can pop the cover, and while it is powered on, determine if there is a particular area within the amp, the vibration is coming from ? If you can, I believe I can help you. Where are you located ( city ) ? According to the web, the Marantz mm9340 weighs in at under 45 lbs. Dynamat, Peel & Seal ( roofing repair tape ) and others can be used to control resonance and vibration within every chassis. I dampen everything, and believe you me, the sq improves every time.

Resonance of cabinets or a chassis makes me shake my head. SO Many times, I have used a vinyl damping sheet to take care of the problem. I just put it on say the top cover of the component. I can use the word drastic here and not be exaggerating, since I will tap a component cover and it will literally ring like a bell before placing a sheet in place. After the sheet is placed, I get a small dull thud.

 In your case, I wonder if this isn't just treating the symptom, since I have never felt a chassis vibrate as drastically as you describe. If in fact if it is a reaction to a magnetic field from the amp, I would think that taking the cover off would expose a large magnetic field to your system components that are in proximity of that field.

Lots of good information here, thanks!

@4krowme I'll get to this tomorrow and check out the magnetic field.   I have a small compass I can use to check the field.  If the resonance is internally generated, removing the steel top plate might squish it.

@mrdecibel The Dynamat is designed for sound absorption damping resonance while Peel and Stick is roofing material and the datasheet states nothing about sound absorption.   Have you found that Peel & Stick is as effective as Dynamat?  Dynamat is certainly more expensive, that is for sure!