Great choice in my opinion - the Baby Sumo is a really impressive implementation of Class D. I've used a Tonehammer with some success, and a Mesa Subway+ preamp and DI - the latter was really impressive, though they're really difficult to find and are no longer in production. I've used all sorts of pres with the Baby Sumo (Warwick PR40, Monique, SF2) and they sound how you'd expect. I think I read somewhere that somebody had experienced some input gain problems with the Sumo, but I certainly haven't encountered any issues with signal. I haven't used an SBP-2, though if it's a pre that you know and favour, probably worth a try - I'm pretty sure the results are what you'd anticipate