I'll say at the outset that I do not think the Pass 30.5 mates well to Zu speakers, so having heard both, I think the SIT amp will be a clear improvement.
The SIT amp is very smooth and articulate. It is among the two or three most natural solid state amps available at any price; and to put it another way it is one of the two or three least objectionable solid state amps you can buy. Within its power limits, it's listenable and satisfying -- if you've never had a better tube amp. Even on Zu at 101db/w/m, the amp's dynamic compression is evident at high but not deafening SPL. Its primary deficiency compared to a range of good tube amps is that while the transient events are defined, impactful and clear, decay is over-damped. You don't get "the whole note," compared to more complete amplifiers. But the SIT amp does have much more tonal depth natural space than the XA30.5 (which is exceedingly clean in its own right).
That's my view of it. If I were committed to solid state for Druids, SIT and Lavardin would probably be my contending choices.
Phil