My MC275 has a clueless sound-change...

oh forgot to mention, I have the current model.
Not the vintage one.

The machine really sounds normal, no pop, no cracking sound, no sparkles in the tube, no abnormal distortion at normal listening level.

*** It will be very difficult for me to get the tubes checked. (If someone in the NYC area is willing to help, I will greatly appreciate it.)

Is the top shelf made of glass ?
That coud explain your desscription of the changes.

If you are driving your 275 through the single ended inputs, the V-1 splitter/inverter tube (12AX7) is IN THE SIGNAL PATH and has BOTH CHANNELS running thru it, making it the MOST important tube for sonics. AND, because that model 275 doesn't have balancing pots on it, it's critical that both sides of the V-1 tube match closely for gain, or you may experience channel imbalance for which (in the Mk-V) you can no longer compensate. Therefore, IMO, you should be using the best 12AX7 or 5751 tubes you can afford for ALL THREE 12AX7 positions, and especially for V-1. The 12AT7/6201 quad of tubes is important for sonics too; not as much as the 12AX7's but in a McIntosh amplifier, more important than the power tubes.

IMO, amplifiers of all descriptions should be either a.) near the floor on an amp stand, or b.) on a VERY STOUT medium height rack (30" max. gives a decent height-to-footprint ratio,) but remember, even an extra 30" of speaker cable could be costly; both in dollars, and in damping factor, if you're using a tube amp ;-)
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I would suggest you put the amp back on the floor to see if the "proper" sound returns. This kind of thing can happen with solid state amps. I have a YBA amp that I can dramatically change the sound on by simply changing the material I use under the amps feet i.e soft vs. hard material. Your amp, because it is transformer coupled, is less susceptible to changes in sound due to long speaker runs. That being said, the rule of thumb is to keep your wire runs as short as possible since there is no such thing as a straight wire with gain. All speaker wires introduce some coloration. The longer the run, the more pronounced that coloration becomes. I always think of speaker wires and interconnects as tone controls. G