One amp has different stenciling/lettering and is likely a different year in production. I used these with Acoustat electrostatic speakers for awhile and the sound was breathtaking. I'm now using a highly efficient pair of horn speakers and I can hear some hum out of the horn speakers that I could not hear from the electrostats.
I sit pretty close to my speakers and the hum is noticeable between songs. The hum does not go up with the volume and remains constant. I never tried using any external ground attempts. I feel these would suit a larger room well.Once they get turned up a bit the sound has incredible 3D realism unmatched to anything I have owned. On one amp one of very largest transformers also vibrates and lets out a hum however not through the speakers. I recently pulled out the receipt for work performed [which is now currently misplaced] and was surprised to read a ground switch was installed inside the amps. I never tried switching the switch to see if that had any impact on the audio hum I hear through sensitive speakers. The rack is not included. My Commitment is to Your 100% Satisfaction! Vintage Treasures At low Prices. I'm human make mistakes and not perfect please give me the opportunity to make things right if not completely satisfied. It is the star system at the bottom of the feedback page. If there is a problem with item please contact me before opening case or leaving dsr ratings! IF YOU ARE NOT PLEASED LET ME KNOW AND I WILL WORK TO MAKE IT THAT WAY. The item "McIntosh MI-200 tube power amplifier mono blocks pair" is in sale since Wednesday, January 22, 2020. This item is in the category "Consumer Electronics\Vintage Electronics\Vintage Audio & Video\Vintage Amplifiers & Tube Amps". The seller is "flukyfinds" and is located in Lake Oswego, Oregon. This item can't be shipped, the buyer must pick up the item.