If I'm correct in that VPI has or will be discontinuing the SDS, it seemed that accepting the ADS was a better option at the time. Again, no one offered an explanation.
I'm not surprised they weren't able to fix the SDS, I don't know who they have working for them that could repair a piece of digital equipment such as that. AFIK, the SDS has been discontinued, but all of the ICs and PCB components are still available; the power supply is a SMPS type and there are many good candidates for replacements if the original is not available. What was wrong with it BTW? I suspect I may be able to fix it via e-mail and some minor soldering.
I'm also not surprised that the ADS is off frequency or that it drifts with time. It uses an analog oscillator which subject to drift with temp and time and will be nowhere near as accurate or stable as the crystal controlled SDS. It looks as if they used the same output scheme as the SDS (small PCB mount heat sinks) so I think the heat build up inside the case will be significant and cause the frequency to drift.