interesting thread. I had an LS25MKII, currently have an LS15 on tap, but have been using a Promitheus TVC. TOmorrow however I get to hook up a BAT VK5i (6922 for current sources and signal gain). I have been a bit mental lately zeroing in on a long term preamp. I suspect the vk5i will be a good match and offer the traits I desire. I like a bit of romance, yet fast, detailed, and powerful with a dense soundstage. The TVC is surprisingly good at most of these things but lacks that last bit of palpable realism (to my ears) capable of the rest of my gear. I really mean the TVC is outstanding though, and especially for the asking prive... I have been listening to it for weeks now and swapping back and forth between that and the LS15 there is no obvious "winner".
However, I suspect that Vk5i will be in another league and I look forward to getting to know it. It has been my experience with the LS25MKII, that, basically it is ruthlessly revealing. Very detailed, yet sometimes painfully illuminating subtle flaws in either the source material or elsewhere... Probably a great tool for a mastering engineer's studio but a bit too much for my tastes (I did not have a go with it in my current system, rather previous incarnations with similiar equipment, it is quite possible that it would be a stunning match with the Pass!). It all depends on your own desires (obviously) and some find great peace with the 6h30 designs (it is probably wrong to stereotype a gain device). I would like to try other manufacturer's implementations on this tube, esp say the BAT Vk51 someday.
Have fun