Maybe it was defective.
I did have so many of these units over the years and when I think about it the SB unit wasn't even a SRD 7.It was a SRD 6.Since then I had avoided the SB types, maybe for wrong reasons, but later I have read about loudness fluctuations in combination with SB units happening to other owners, too.
Do you know why Stax had decided to sell non-SB units? There must be a technical reason since the SB types are cheaper to produce.