ARC LS25 or Pass Labs X1??

Sonics wise, the X-1 is an excellent pre-amp and perhaps one of the best in it's price range. The only things I did not care for were the ergonomincs (a rather clumsy remote and interface) and lack of finer increments on the volume control.

And with proper line-conditioning the X.1's separate power supply with the 6ft umbilical cord is essentially an unnecessary extra component to shelve.

I've always used the same line conditioner with all the preamps I've owned which also included the X2.5. In my system, the sonics between the X.25 and the X.1 were near identical.

-IMO

The ARC LS25 MKI with NOS 6922 tubes is an excellent preamp

I can only offer my experiences with the ARC gear as I have not heard the Pass pre amps.

The LS16 actually has a higher gain output of 12.2db in SE config. The 25 is adjustable from 0,6,12db Se.( 6,12,18db in Balanced) Your 15 runs 6db in SE out,so the 25 would be a good choice, run at the lower 0db. setting.. That said , I dont think that the 25 offers any more flexability with volume as the stepped attenuator is much as the 15 is, however the lower gain setting will be useful for you here.

Sonically, the 25Mk1 is a nice improvement over the 15 in every area, and cost about 2.2K used. compared to about 2.9K for the used Mk2 which incorporates the 6H30 tubes over the 6922's. For me, I prefered the Mk1 over the Mk2, as it had more airy mids and highs but slightly lighter bass than the Mk2.Not a big difference here, but worth noting. I thought the 6H30 tubes were almost too "accurate" sounding, they are very linear and provide good dynamics, but the Mk1 was able to bloom more in the mids..

Forgive me for not responding to the question you posed ("ARC LS25 or Pass Labs X1??"), but consider the following:

I had to sell a CAT Mk. III preamp because of its high gain which, combined with the sensitive inputs of my VAC Renaissance 140/140 Mk. III monoblocks, likewise gave me no play in the volume control. I have since had a Hovland HP-100 preamp and currently use a Rowland Coherence II preamp, both mating very well sonically with the VAC amps. Both have much lower gain than the CAT (which has nearly 30 db. of gain). The Hovland had no remote. The Rowland has 99 volume steps, a good remote control and gives all the control over volume one would ever want. Rowland's Synergy IIi preamp is close in performance to the Coherence II and can be had for roughly $3k used. Both Rowland preamps are true, fully-differential balanced designs (they feature balanced inputs and outputs only).

Because I believe that the differences in sound between good preamps tend to be subtle (with, very generally speaking, tube designs having an advantage in soundstaging and solid-state designs being more quiet), perhaps you would want to widen your list of potential candidates. In this regard, you may wish to add the Rowland Synergy IIi to your list (as far as fully balanced preamps go, it is certainly a major player).

PS - I have owned an ARC LS-3 preamp for ten years (it still anchors my second system).

I would recommend the Placette Active. If that's not within your budget, the Placette Passive is wonderful also. They both have remotes and very accurate volume controlling which fits your criteria. The owner, Guy, can even adjust the gain for you.

I haven't heard the the two you are asking about, but I feel you should give the Placette a try.

The X-1 is one of the best sounding SS preamp that I have heard. I compared it to many tube, SS and passive preamps. That being said, the mid range is not as sweet as a good tube preamp. The X-1 bettered most tube preamps in the dynamics and low end department. So it comes down to a matter of preference. The new Sonic Euphoria passive preamp is a real sweet preamp for the price. My amp is set to low for a match but it was a really good performer for the price. The Placette active will beat the SE but for much more money.

Happy Listening.

Since you already have the VAC amps, why not go with a VAC Renaissance preamp? It has a motorized infinitely variable volume control that is a joy to use. I replaced an LS15 with a Renaissance preamp and the the difference was far more than I expected, better low end extension, quiet, goodbye grain and hello ambience plus gorgeus to look at. If you like your amps this is your best choice. I hate stepped volume controls and any hum heard through the speakers and this preamp will eliminate both.