Audio Research LS-5 & Pass X250, good match?

Looking to upgrade my X2 preamp. It's been a great pre but would like to delve into tubes. My speakers pretty much preclude the use of tube amp, unless it's very high power & $$$. Wondering whether the LS-5 would be a good match (impedance issues, etc) with my Pass X250. Both being balanced configuration & hearing great things about this pre I am curious to try. Also knowing the LS-5 has 10-16 tubes, how much maintenance & expense is regularly required on this piece? I know the Mk II has less tubes than Mk III, is there a pronounced difference? Thanks.
The LS5 mk1/mk2 have 10 tubes each while the mk3 replaced
the 2 input tubes with FETs and count went to 8. The mk2/mk3
use the 6922 tube while the mk1 uses both the 6922 (6) and
12bh7 (4) tubes. The REF1/REF2 preamps are very closely
based on the LS5mk3 circuit.

The swap of the input tubes for FETs in the mk3 definitely
resulted in less noise over the mk2 version but many feel
(myself included) that the mk3 lost some of the mk2 bloom
and is slightly more sterile but I feel it is still very
very good. I've had an LS5mk2 for almost 9 years now and am
still absolutely in love with it. I have fairly low/medium
sensitivity speakers at 86db so the tube noise floor was
never an issue for me. I could never hear it even right in
front of the speakers. Others with high sensitivity
speakers found the mk2 tube noise floor to be basically a
minor annoynance during silence but rarely if ever an issue
during music. Similar to analog tape hiss. These were the
people that seemed to benefit most from the mk3 upgrade
which lowered the noise floor a bit.

I would HIGHLY recommend the LS5 as it should have no
problem driving your Pass. I vertically biamp two ARC
D400mk2 200wpc solid-state monsters with my Ls5mk2 to great
effect. The tube/SS synergy is just awesome. Anyway, if you
have higher sensitivity speakers you may want to consider
the mk3 version. Otherise I'd go with the mk2.

As a long time LS5 MK II owner (7 years) and MK III owner (a little over 1 year), my experiences with the 2 versions is different than the previous post. In 1997, I took my MK II to the ARC dealer to compare with the MK III. It was the LS5's incredible midrange bloom that attracted me to this unit in 1994. Upon hearing the MK III, I heard no reduction in this quality at all. What I did hear was a much more coherent tonal balance with the MK III in that the top end was much more natural; the MK II can be a little fatiguing once you hear and get used to the MK III. And the MK III clearly has greater resolution; you simply can hear more detail. The MK II did however have a little more punch and extension in the low frequencies; whether or not this too was natural I did not know. The III was clearly a refined product, but from this experience, I did not think the $1500 upgrade price was a good value so I kept my MK II.

Five years later, a local audiophile had his MK III for sale. He came to my home with his MK III and we did extended listening between the two. Amazingly enough, as I experienced before, their tonality is nearly identical except for the rising top end of the MK II. And again, after hearing the MK III, going back to the II clearly showed how fatiguing it can become once you hear the refinements of the MK III. I bought his MK III and then sold my MK II for just a little less so I did well. The listening sessions between the II and III used ARC VT130 and Classic 150 amps driving Magnepan speakers. So if there was fatigue from an overly forward presentation with this setup, I can only imagine it being more prominent with SS amps.

Unless you truly need the 30db gain of the MK II with low output phono cartridges, you really need to consider a MK III. There are plenty of higher gain phono stages these days such that 30db in a line stage is just not needed. And this really is too much for CD playback anyway. And the 12db setting in the MK II I found to be too degrading so I always ran my MK II in the 30db setting for Phono and CD. The III comes either as 12db or 18db and can be converted either way with a resistor kit from ARC.

A year later I discovered the BAT 31SE which goes far beyond the MK III in refinements than the III did over the II. As much as I was a huge LS5 fan for so many years, the 31SE is in a completely different league in every way..and it gives up nothing in the magical bloom and 3-dimensionality.

Warning: I see LS5 units for sale, often claimed to be MK III but they are clearly not. The MK III does indeed have only 8 tubes....AND....the 30db/12db switch on the front panel of the MK II was changed to be a Bal/SE selector on the MK III. So make sure you ask the seller if it has 8 tubes and that this labeled switch matches.

SE-to-Balanced adaptors are still required for single-ended sources on the MK III, but it appears that a conversion circuit (phase splitter) inside the MK III creates the 2-phases from the single-ended source. With the MK II, when using adaptors, only one phase within the unit is being used and the other's input just grounded. Still, this works mighty fine so I would not consider it an issue. But balanced sources are what this unit was designed to accommodate. And adding the BL1,2,3 conversion boxes to these models only destroys much of their magic.

Hope this all helps a lot.

Meisterkleef & Jafox thank you for your hard earned knowledge. Guys like you make A'gon a treasure for sure.