For the UMIK-1, I don't see a place to adjust sampling rate. I'm running it on a windows 10 laptop, and under sound properties I can't find anything adjustable. Way down under properties the default format is 24 bit 48k Hz but it's not changeable, but sounds like it matches what you suggest.
Honestly I like the RZ50 generally and not all that anxious to change except -- it just doesn't work. Something is "stuck", almost like a weird crossover setting, except those are normal (and I think ignored for the tests).
The Umik-1 in fact would default to 48 kHz because that's its limit, but in Dirac, there may be a place where you can set sampling rate, I am not sure about that because mine is a PC standalone version, so you have to dig in and see if Dirac/AVR has such a setting, if not then there is no issue, just ignore what I said. If there is, then make sure you set it to 48 kHz.
I have had many AVRs/AVPs over the years, and have never experience any such issues, so may be you should try a factory reset to the original out of the box setting and see what happens, nothing to lose to try things, if you are waiting for a resolution from your dealer anyway.