Oscillating Preamp?

[Opti-cal]

Also not helping is my speakers being close to 100db efficiency. . . . I hear everything!

[anthonyTD]

Understood,
Ok, perhaps i can look through my stock and find another RCA for you, if you would like my help, please get in touch via PM, or directly through my site, and i will see what i can find.
A...

Attachment 27220

Preamp is a Musical Paradise MP-701

Offending tube is at the rear. I have rotated it and the issue occurs on the other channel.

I have to use it in the main driver position (top or bottom) as the other tubes are Sylvania's (now Gold Lion's) and this configuration sounds far superior.

If I had another good RCA 12au7A I'd just swap them and I'm sure it would be cool!