AR LS 26 and Pass Labs X350.5 Compatible?

tried ref 3 & 350.5. Very good combo, no problems.

Aurelius, best is to try.
Dave(Dpac996) makes an excellent point and provided a good bit of information that should hint you on what to look for and what to ask. Call ARC and ask them all the questions you already have just to put my the mind at ease.
If you are settled on Pass X350.5, then when you buy it, try to get an LS-26 as a loaner to try out at home.
You may love it or the preamp may simply be not your sonic cup of tea. That would be a problem as well. So try before you buy, if you can of course.

As an aside, before I bought my X250.5, I called Pass Labs and also spoke with Desmond and asked him if my ARC LS-25MkI would be a good match with X250.5 and I provided all the specs to him. He said it would be no problems and he knows of many ARC tube pre/Pass amp combos.
I took his word for it and 15 months later I am still using and enjoying the very same combo and am very happy with the way my system sounds.

But do your research and make all necessary phone calls to confirm things with proper authorities(ARC and Pass Labs) to be more or less certain there will be no flukes.

I think there's sufficient end user confirmation and information from the manufacturer to resolve the OP's question and ignore the guesstimators.

To correct an earlier post, all Pass Labs X.5 series amps have a balanced input rated at 22K not 20K.

The XA.5 series' balanced input is a bit higher at 30K.

Bob,
I am somewhat troubled by those findings as well, but when you realize the output capacitance it has to be that way. But it seems odd that ~18k for a sota preamp... this should not be the case. To get the most potential from your system specs like this sort of force you to use amps with at least 50K ohms nominal Zin. There is no getting around it.