Tube preamps are often used with SS amps with great success.
Some people prefer an all tube system and some don't.
Some people prefer an all tube system and some don't.
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.
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.
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.
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.
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?
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!