I've been away and only just saw this, so perhaps you've already gone with the Brystons (not a terrible choice!)...
For surrounds, the truth is they're underutilized compared to mains, so while a perfect match to the front soundstage is ideal, it's also not something I've ever felt beholden to. In my own setup I went Class D, largely because the surrounds are spending so much time idling or at very low power that efficiency felt like a good trade. If you don't want to do that, no worries - Class AB obviously still gets a long way down that road. I would suggest looking at ATI amps. They have both Class D and Class AB models with beefy toroidal power supplies and plenty of capacitance. They aren't too well known on their own, but do lots of OEM work for other brands on the AV side of the industry. In the Class AB realm, the 4000 series has plenty of power for surrounds and checks enough audiophile boxes that I considered them for front channels once upon a time. The 3000 series has even more grunt (probably overkill) at a slightly lower price point. And the 1820 series is a more cost effective route but, in my opinion, still plenty for a surround or distribution amp.
I've found that matching the voltage gain is particularly important for surrounds, frequently even more so than raw power. I actually custom-specified the gain for my theater's surround amps to ensure a consistent volume ramp with the mains. In your case, if using the Boulder for fronts, I'd be tempted with an ATI 4005 - it checks a lot of the boxes I'd personally be considering if I were sticking to Class AB for a multichannel amp.
Full disclosure: my theater amplification is from Apollon, using a mix of Class D NCore NC2000 (front) and Icepower 1200AS2 (surround) modules. I use ATI in my 2-channel setup, and have never felt myself needing more, despite the relatively modest price of admission.
