I agree that the Bryston 28B SST2's input impedance may be challenging to some preamps but specs tell only part of the story. My previous preamp, the Jeff Rowland Capri S2 is spec'd at 80 ohms. While this is more than the 20 times rule for the Brystons, the Capri seems to struggle and the volume setting has to go to almost 80-90% of max to achieve satisfactory levels. When stretched to this point, the Capri begins to sound strained and compressed. That said, my speakers, Vienna Acoustics' The Musik need lots of power to sound good at healthy SPL's. I believe the Maggies need just as much power or even more.
After switching to my current preamp, the Ayre K-5xeMP, which is spec'd at 50 ohms output impedance, I am not sure whether the 30 ohm difference is significant but my ears tell me it indeed is, and very much so. With the Ayre in the chain, music flows effortlessly, dynamics are fabulous with no compression. And I very rarely need to go beyond 50% of max volume setting for everything to sound "just right".
But to reiterate what has been repeated too often, try before you buy, if you can, that is. Sometimes, a home audition with a side-by-side comparison within the context of your own system, may be luxury that is hard to come by.
Cheers! J.
