Just cured this exact problem (Atma-sphere MP-3) with tube replacement. Same family of tubes...mine were 12AU7's. I highly recommend N.O.S. Make sure that wherever you get them from, you have a return privilege, in case they turn out to be noisy or microphonic. You'll know this immediately! For more help, you can e-mail me. Happy Tunes!
Tube Question
I recently installed an Air Tight ATC-1 preamp into my system. I noticed that the left channel was considerably noisier than the right but not so noisy that I couldn't live with it. Very recently, however, there is a low level, high pitched squealing sound from that left channel. Is this due to a bad tube? The line stage uses one 12ax7 for each channel. Thanks in advance for your help.
Jon
Jon