I would think the only reason to interject a preamp between your source and the Brystons is to better match impedance mismatches. If the source can readily drive the input impedance of the Brystons to full power, then I am not sure I see the advantages. The input impedance of the power amp will be about the same as the input impedance of any pre-amp, so from the source point of view, its the same load. Perhaps with a pre-amp, the source will need to produce a lower voltage range (because of the added gain of the pre-amp), and this could give some sonic advantages.
For example, the power amp might require a voltage of 1-2 V for full power, whereas the same 1-2V could be derrived from the pre-amp with the source providing only a few hundred millivolts. If the source uses IC OPamps, then the lower the outpout voltage the better. If the source has a class A discrete output stage, this would be less of a factor.
I used to use a CA 840C to drive my power amps directly, but I used a passive attenuator for gain control. So this placed a resistor attenuator between the power amp and source, which is a compromise (impedance/load) at best. I discovered the attenuator (5K) was way to low for the OPamp output of the CA 840 to dive properly. Sound was clean and detailed, but significantly devoid of bass response.