I'm your man on this one. I have two USA made Ampeg B2REs that I use (almost) exclusively for their power sections (I consider their kinda sterile sounding built in preamp more of a spare than anything else). I activate the power section of my B2REs through the Power Amp In patch point on one of them and through the Effects Return patch point on the other. If patching via the fx return colors the sound, I can't hear it. The only difference I can discern is that patching your external preamp via the Power Amp In overrides ALL the controls on the B2RE whereas patching via the Effects Return allows the B2RE Master Volume to remain enabled. I exploit this by plugging my Alembic F-1X (which has no master volume) into the Effects Return on the B2RE which enables me to run the F-1X volume control at the level that yields the tone and/or DI level I want yet I'm able to control the overall volume with the Master Volume on the B2RE. I have my other B2RE in a rack with an Ampeg SVP CL and an Aguilar DB659 preamp, both of which have gain/master setups so I plug those two into the Power Amp In jack on the B2RE. One fine point to consider when using the Power Amp In patch point is turn off transients. I've found that with my non-Ampeg preamps, I'll get a fairly loud (and I assume damaging to the speaker) turn off thump sound when shutting down both the external preamp and B2RE at the same time via a power conditioner whereas this will not happen when using the Effects Return patch point with the Master Volume on the B2RE set to zero upon turning off the entire setup.
A (now defunct) club I used to play at regularly with several bands had a nice house PA and provided a backline as well. The house bass amp was a Mesa Boogie 4x10 powered by an Ampeg B2R (the 100 watts less powerful predecessor to the B2RE). Anyway, a bass buddy of mine played there a couple of nights before I was to play and told me that the preamp section on the house B2R was acting kinda funny so I brought one of my mini racks loaded with an Ampeg SVP CL and an Aguilar DB659 and used the SVP CL that night. It sounded really good and from then on out, I brought my own preamps to that gig and vowed to find myself a USA made B2RE (I have like zero faith in Pacific rim Ampeg gear after all the poor reliability war stories I've heard on this and other forums). Luckily, these gems (at least from the standpoint of their power sections) seem to be poorly regarded, readily available and cheap (I got one of mine for $200 w/SKB 2 space rack and the other one for $150). It's nice to see that the "secret" is getting out on the use of the B2RE as a power amp.