The xf2s have entered the building. I'll reserve final judgement once I've had them for a while, but first impressions put them to the top of the ones I've tried.
One slightly disconcerting thing is that one of the tubes has small flakes of some form of debris on the inside of the glass. These were visible on the ebay photo, but I assumed that it was packing dust on the outside. Oh well, the vintage route is not without risk. Doesn't seem to affect performance, and the balance/offset settings for the xf2s were in the same ballpark as for all my other tube changes.