It started out some months ago with the sound breaking up in one channel. Then, the sound became more or less balanced, with the problem in both channels. This would tend to indicate some kind of bias problem. Yes, they become less loud as time goes on, compared to the efficiency when they worked correctly.
However, if the headphones respond differently to a known good quantity i.e. the SRD 7SB, which I know is good, it would indicate the problem is with some kind of time dependent short in the headphones somewhere, or a poor contact etc. etc.
I.E. bias charges, stators sound fine. Over time, stators sound worse, plugging it back into known good bias source 7SB, sound is worse. Sounds like a scratchy, intermittent low quality transistor radio, not Stax.