Stickman, often (although not always) the number provided in specifications of balanced input impedances represents the sum of the input impedances of the two input signal "legs." An RCA-to-XLR adapter routes the signal on the center conductor of the RCA cable into one of those two legs (usually pin 2) while connecting the other one (usually pin 3) to ground (pin 1).
So in those cases the unbalanced output of the component providing the signal would see a load impedance that may be in the vicinity of half of the specified input impedance. Or, if not half, a load impedance that is substantially less than the specified balanced input impedance.
The Bryston amplifier you were considering appears to be an exception to that. While there are some inconsistencies between the input impedance values shown in the manual and the brochure, and the values that can be calculated from the schematic, it appears that they are defining balanced input impedance as being the input impedance of each leg. And that value is in the same general area, 15 to 20K, as the unbalanced input impedance. So the preamp would see a fairly similar load, and perhaps an identical load, with or without an adapter.
A separate question, apart from impedance considerations, would be if an amplifier having both balanced and unbalanced inputs would in itself sound better, the same, or worse using the balanced input with an adapter vs. the unbalanced input. Any of those three possibilities is conceivable, IMO, depending on the particular design.
Regards,
-- Al
