UPDATE 1 (copied to the OP)

Amazon had the XS on for £393 ($474) with free same day delivery so I couldn't resist. I'm a firm believer in the burn-in theory, especially so with these as it is actually recommended, so they're going into a drawer with an old mobile phone playing a radio station for a few days before I make any judgements.

The BTR5 can drive them in SE, but only just. I can easily take the volume up to max and still listen comfortably so, given that the Qudelix is often mentioned in the same breath as the XS, I've ordered one and a 2.5mm balanced cable. Hopefully they'll arrive by the time the burn-in is done, then I'll let you know what I think.