AR LS 26 and Pass Labs X350.5 Compatible?

Hello to all the audiophiles here on Audiogon. Being new to the high end of audio I was wondering if some of the more knowledgeable experts could give me their opinion on something I've been thinking of.

Recently, I upgraded my venerable Ohm 4s to the 4.5 S3 and I wanted to pair it with an Audio Research LS 26 and a Pass Labs X350.5, both of these choices after reading some of the forum opinions here.

I contacted Pass Labs and a gentlemen named Desmond said there would be no problems with the pairing. However, after reading some opinions on mixing solid state amps with pre amp tubes would they really be compatible or would there be a better alternative? Thanks in advance for any replies!
Tube preamps are often used with SS amps with great success.
Some people prefer an all tube system and some don't.
Aurelius, I have a Audio Research Ref3 and had it paired up with PASS X-600.5's and that was a great combo driving Eggleston Andra 2's.
One thing to consider is that the Pass amps have a relatively low input impedance while the ARC gear has a pretty high output impedance. Though I cant verify this, I had the nagging feeling that my Ref 3 and XA160 was not a good combo because of this mismatch. I have since changed my pre and amp to ones with a higher ratio of output Z and input Z. I am now very happy with the sound though I cant be sure if it is because of the impedances.
The thing that would be nice to know, to give a better feel for this whole impedance gig, is output impedance vs frequency. All too often manufacturer's supply a single figure for output impedance for say, a tube preamp. It's usually at 1 kHz,and it may be 500-1000 ohms typical. However as 99.9% of the tube preamps on the market implement capacitors to block the high voltage DC, and that good sounding caps have to be a finite size, the output impedance for the typical 6922 or 6H30 design can really rise to the k's of ohms in the lower frequencies. It would be rare for a manufacturer to outright tell you its going to be a bad match, and i'm not sure of the LS26 output impedance across frequency, but into 22 kohms, you will have bass issues if the output impedance is in the thousands of k ohms. Again I have no idea what the ARC is; they might have that data at Pass and can therefore make these recommendations. I know that my BAT Vk5i did not have the energy in the lower octaves as a much simpler solid state source did. I think Bat's do rise way up there in the k ohms region around 20 hz...someone please correct me if i'm wrong...this is not going to be an issue if you are driving a BAT amp (100K) or say 50 K, but the amps that fall into the 22kohm input impedance (come to think of it, this figure, since it is impedance implies frequency dependent variation and it can actually be lower than the specified amount, again why it can be a total crap shoot for brand mix and match)...
Generally the use of this single impedance figure is about as useful (useless?) as damping factors measured at one frequency (pretty much the same thing).
All you can do if you are mix and matching amp and preamp is, if you have not heard the combo elsewhere, pick them so that its not hideously out of whack and simply try it. If it floats your boat you will know it.
Good luck!
The LS-26 output impedance (700 balanced, 300 single ended) will have no problem driving the Pass X.5 series which are 22K balanced and 11K single ended.

Listen to the folks at Pass who know their products better than the guessitmators here.
I am using an ARC LS26 with a Pass Labs X250.5 amp and it is a great combination. I also tried the REF3 and that sounds even better, as expected. I tried using the Aesthetix Calypso with the Pass and that was not a good combination. It worked but did not have the magic of of ARC/Pass combination.
I forgot to mention that I have an X250.5 (same input circuits/same drive impedance give or take as the X350.5).

For the record, let me state that these single data points are pretty useless. They are mostly marketing figures. Think about it folks. Impedance is a frequency dependent resistance to current. On the one had too much data can lead to analysis paralysis, but otoh, too little can lead to bad outcomes. I really appreciate the measurements that stereophile presents when they do equipment reviews. They pretty much cover the spectrum. Of course the final proof is in the listening, and no one can predict that.
Audiofeil, unless these folks have the data in question they are also taking a guess (albeit an educated one at that). Don't get me wrong I love pass products and the company is excellent, but it is what it is.
With the specs Audiofeil quotes, you should not have any compatability problems regarding impedance issues.
I have the ARC LS-25 Mk I and Pass X350.5. I remember reading somewhere before about the impedance matching issue between these two components, but I could not find the threads again.

I just did some research and this one talked about the impedance matching:

Per the manuals ARC LS-25 Mk I has output impedance of 650 omh (balanced), and the Pass X350.5 input impedance is 20k ohm (balanced). Per the above thread, the ratio should be at least 20x. The imput impedance of the X350.5 is at least 30x of ARC LS-25's output impedance (20k ohm/650 ohm = 33.8), so it should be OK.

Dpac996 is absolutely correct. Ask ARC what happens to the output impedance of the LS-26 below 1KHz. Asking Pass about another manufacturer's product doesn't make sense.

It's not uncommon for tube preamps to mate better with tube amps since they typically have large input impedances.
Thanks to all who replied to my thread. I will try to investigate more thouroughly and see what Audio Research has to say on the matter but according to Dev, Joey54, and Audiofeil, it does look promising.
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.
There's a classic example in the current issue of Stereophile -- BAT Rex preamp, $18K. Its output impedance is listed as 200 ohms. Sounds great, right. Should work well with any amp, even Pass Labs and Bryston. Then read JA's measurements. The output impedance varies as a function of frequency, just as Dave stated: 1000 ohms at 20KHz, 415 ohms at 1KHz and 4,800 ohms at 20Hz. So to avoid rolling off the bass, the amp's input impedance needs to be at least 10 * 4800 = 48K ohms.

Will the bass roll off be audible? Yes, but you probably won't notice it unless you can compare it to a setup that doesn't roll off the bass.
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.
Bob, that x10 multiplier "rule" is not something I put much faith in. To actually calculate the drop in dB for this kind of case you simply apply the voltage divider principle at a specific frequency. For example if at 20 Hz the output impedance (what the up stream impedance looks like to the downstream element through same ground return) is 4,800 ohms, and the input impedance at that same frequency is 22,000 (it could actually be different, these are frequency dependent) you get a basic voltage drop of 22k/(22k+4.8k); if it is 1 volt pre divison we are talking 1 v x 0.82, or in dB a -1.7 dB loss. Audible? probably not a whole lot, but that is using these figures. Personally I would rather not like to incur these losses when spending this kind of bread. I would hope that after spending thousands of after tax dollars my electronics can stay razor flat between 20 Hz and 20 kHz. My speakers and room interact to mess the rest of it up but nothing is perfect, eh?
Dave, I think it raises the issue that manufacturers design a family of products. BAT's Rex preamp will work very well with BAT amps. That's all they should really be concerned about. Folks that mix and match manufacturers or tubes and solid state need to be aware that just because two components have matching connectors doesn't mean they're compatible.
Dave, what's the voltage drop if the amp's input voltage is 50K instead of 22K? The ratio becomes 0.91 or -0.82dB. So maybe the 10X rule isn't quite good enough in general.

I agree with you -- I expect electronics to be perfectly flat. Why accept anything less?

And you're correct that the speakers and room are going to muck up the sound several orders of magnitude worse than the electronics. So if that's true, why worry about preamp/amp electrical incompatibilities? Because it allows you to focus your attention (minimize the variables) on the real issue: speakers/room.
Makes you want to get an integrated.
After contacting Audio Research Corporation, they said they are "reasonably sure" that the Audio Research LS 26 could drive the Pass Labs X350.5 with an input impedance of 22 kohm balanced with no problem. They also said that although they would have like to see the input impedance of the X350.5 at 30 kohm, there would be no problems with the bass rolling off as some have suggested. Thanks to all who have given their expertise on this matter!
so what preamp are you planning on getting?
For now the ARC LS 26 is on my hit list for an audition, having a nearby dealer and thats where I'll start. I 'll keep my options open for other preamps by reading this forum for ideas.
Any news on the ARC/Pass front?