I’d approach pairing a passive preamp or other unity gain preamp with the XA25 with caution, as its gain is somewhat on the low side (spec’d as 20 db; measured by Stereophile as 20.4 db). Stereophile measured the XA25 as being capable of supplying 80 watts into 8 ohms. It can be calculated that for an amp having a gain of 20.4 db to be driven to an output of 80 watts into 8 ohms an input of slightly more than 2.4 volts is necessary. Without a preamp supplying additional gain you won’t get there with the vinyl-based source you mentioned in the other thread, even with the volume control at max. And even with a digital source you would find yourself setting the volume control at or near max to drive the amp close to 80 watts, should that be necessary on the musical peaks of some recordings. That being especially likely to be an issue with recordings having wide dynamic range, such as many classical orchestral recordings.
On the other hand, though, with your Zu speakers you may never need the amp to supply anything close to 80 watts. But I would consider this issue carefully before proceeding down that path.
Regards,
-- Al