... perhaps Al and/or Ralph can speak to whether inserting an impedance buffer between the pre and amp could compromise the signal.
Certainly inserting an additional active stage into the signal path to correct an impedance incompatibility can invoke some tradeoffs, to a greater or lesser degree depending on the quality of the buffer.
But although nothing in the preamp itself would be changed, I would view doing this as in effect amounting to implementing a design modification to the preamp, with the modified "preamp" consisting of the combination of preamp + buffer stage, and a (short) pair of interconnecting cables between them. (It would most likely be best to make the cables on the input side of the buffer as short as possible, with those on its output side as long as necessary, so that the longer cables are driven by the device having low output impedance).
Viewed in that manner, it seems to me that even apart from resolution of the impedance incompatibility, the subjective effects of adding a buffer stage would stand a reasonable chance of being an improvement, rather a degradation, although either outcome is certainly possible.
In any event, the OP indicated in his other thread that the opportunity he had to purchase the Bryston amp went away, so the question may be moot.
Best regards,
-- Al