Cary SLP-05 with Pass Labs X-250

George, per Stereophile’s measurements the 400 ohm spec is not accurate:

The SLP 05’s output impedance is specified as a usefully low 400 ohms. However, I got significantly higher values for the unbalanced output of 1500–1600 ohms in the midrange and treble, rising to 3400 ohms at 20Hz, with similar if slightly lower figures for the balanced output. The Cary preamp needs to be used with power amplifiers having input impedances of 10k ohms or greater if the bass is not to sound a little lean.

... And given those measurements I would consider JA’s suggestion of a 10K minimum load to be optimistic.

That said, the 22K **balanced** input impedance indicated in the manual for the X250 would seem to be a fairly reasonable match for the SLP-05 in terms of impedance compatibility, given also that the SLP-05’s output impedance has much less variation over the frequency range than in the case of many other tube-based preamps.

Regards,
-- Al

This is very interesting to me. I owned the Pass Labs 250.5 for a short time, and really disliked it's sound with 4 different speakers. I was using the Morrison Elad preamplifier with it. Now I'm thinking that it may have been an impedance mismatch after all. Any thoughts Al?

Hi John (Roxy54),

No, I don't think there would have been an impedance compatibility issue using the Morrison Elad with the X250.5, or with most other power amps.  See the info provided in the PM I sent to you on 3/3/2017.

Best regards,
-- Al

Thanks Al. I guess that just wasn't the right amp for me. 

However, I got significantly higher values for the unbalanced output of 1500–1600 ohms in the midrange and treble, rising to 3400 ohms at 20Hz,

Almarg is right, I got the 400ohms from the Cary website.
This is why Stereophile is such a good site for seeing what's really going on with products, as their measurements can't be disputed.
In this case your Cary is not a good match for the Pass X-250. either get a pre with low impedance across the whole audio band, or get an amp that has 50kohm or more input impedance. 

Cheers George