Pass Labs & Belles class-A amplifiers

According to the Pass Labs website, the XA-.5 series amplifiers have input impedance specs of 30K ohms balanced and 20K ohms unbalanced. The May 2009 Stereophile review of the XA-30.5 states an unbalanced input impedance of 15K ohms. I don't know where the discrepancy in the unbalanced input spec originates, but John Atkinson's test measurements showed an unbalanced input impedance of 20K ohms.

My XA30.5 defintely said it was 15kohm unbalanced, maybe the monoblocks have higher impedance? I would ask Pass Labs. If it is 20kohm, slighly better yet for tube pres, but I do think you use many tube preamps even with the 15kohm rating if that is correct. I talked about this with Pass Labs before buying the XA30.5 as I knew I would be using a tube preamp, and I remember feeling comfortable that I would not have any audible problem with my preamp at the time, the Joule LA-150 MKII.

Again, the specs on the Pass Labs website to which I linked are 20K ohms unbalanced for the XA-30.5, and was corroborated by John Atkinson when he tested the amp.

The manual, see page 12

http://www.passlabs.com/pdf/product%20manuals/xa30_5-om.pdf

says 15kohm, and also my recollection in discussion with them. The XA60.5 manual says the same thing. But, either way, I still think it will work fine with most tube preamps with under 1000 ohm output impedance.

I suppose only a call to Pass Labs will answer the question, which seems ridiculous for a company known for its excellence.