Mechanical Hum from Class A Amplifier Clayton

Do you run them both off the same circuit? If so, then any residual noise or mechanical hum/buzz should be the same (w/in normal piece to piece variance). If they are not on the same circuit, you may have some DC offset on the circuit w the noisier amp. Class A vs AB should have nothing to do w it, other than a hot/warm idling Class A amp may put more thermal stress on internal components and cause faster wear.

Large torroids are susceptible to mechanical noise. The torroidal transformers and capacitance of these amps are huge. The M300s are fully balanced so the first thing is to give them a balanced input signal. A single-ended signal with rca-XLR adapters will not work due to ground issues. If you are coming out of a single-ended preamp you will need to use Jensen rca-XLR transformers, which work great and are very quiet. Do not use a power conditioner - run them straight from the wall. If you still have noise I suggest contacting Wilson Shen (owner/designer at Clayton) directly as he is one of the nicest people I have worked with in audio. He does offer a power supply upgrade for M300s that he did to my amps. I believe the upgrade includes better torroids, increased capacitance and other improvements. The result was an improvement to the already great SQ and elimination of any mechanical hum. Unfortunately the upgrade was not inexpensive but I am happy to own the M300s because I have heard nothing SS that sounds better (although there are many I have not heard).

my Preamp is fully balanced, Even when no source is connected the amplifier is very noisy. I am speaking to Wilson now probably going to send them back. Mines were purchased this year and came to me with a fault, I shipped them back to Wilson for repair and it came back faulty and noisy again. I assume its the power transformers are not designed well for the 220-240 voltage.

With speakers as sensitive as yours, a bit of noise shouldn't concern you too much. The difference in the heat output is a concern and should be looked at. Probably just a bias setting out of adjustment, but worth having it looked at.

Good points Timrhu, my speakers are only 90dB sensitive and need the high power to drive less than 4 ohms impedence at lower frequencies - I missed how sensitive the partnering speakers are. While these are great sounding amps in my system, Clayton makes smaller amps that may work better with such sensitive speakers. Regarding temp., here is a link to an interesting post where someone measured their heat sink temperature over time to document how long it took a pair of these amps to achieve full operating temperature
http://www.audiocircle.com/index.php?topic=101551.msg1026780#msg1026780