In the manual for the 4B3 the spec for BALANCED INPUT IMPEDANCE states:
NON-INVERTING: 20k ohms
INVERTING: 6k ohms
In comparison, my previous 4BSST2 was 20k ohms for both +/- inputs.
I had a Pass Labs X2.5 preamp with about 1k output impedance (both legs). I used the combo in balanced operation and the match with the 4BSST2 seemed to be excellent as the bass extension and control was thunderous and full. I noticed no obvious (or even subtle) impedance mis-match issues.
I recently picked up a Pass XP-10 preamp with about the same Zout: 1k balanced. (single ended out is about 150 ohms, much less!)
I am concerned about the low 6k Zin on the INVERTING path; as you likely know an audio rule of thumb is minimum multiple of 10x; better still 20x on the Zin/Zout ratio. With the Pass XP10/4B3 we see 6k/1k =~ 6.
I am currently using the XP10 to the 4B3 single ended; the results are wonderful, but I am waiting for longer (8 foot) balanced cables to arrive. I am curious to hear the results.
I asked Bryston 2x via 2 different methods and received no response regarding the spec; I was curious why the INVERTING input is less than half that of the NON-INVERTING leg, and more importantly if Bryston had any concerns about using a preamp with a high-ish output impedance (balanced). Could there be an error in the spec and both are really 20K?
I appreciate in advance any advice/insight!