Jim, I agree with Lowrider and Soix that one of the first things that should be done is to try different 12AU7's in your Sphinx. FWIW, among currently produced tubes I've had excellent results, although in a completely different application, with the Genalex Gold Lion reissue.
Also, given that the Sphinx provides a headphone output (which I note BTW does not have the two tubes within its signal path), it may be helpful in diagnosing the perhaps multiple contributors to the issue to purchase an inexpensive pair of headphones. The Sony MDR-7506, which I own and which is widely used in pro audio and video applications, is surprisingly good in relation to its $85 price. Although as might be expected there is no comparison to my Stax's.
Good luck. Regards,
-- Al
Also, given that the Sphinx provides a headphone output (which I note BTW does not have the two tubes within its signal path), it may be helpful in diagnosing the perhaps multiple contributors to the issue to purchase an inexpensive pair of headphones. The Sony MDR-7506, which I own and which is widely used in pro audio and video applications, is surprisingly good in relation to its $85 price. Although as might be expected there is no comparison to my Stax's.
Good luck. Regards,
-- Al