Stereo pre-amps with dual outs for bi-amping and a subwoofer line out

I think the McIntosh c47/c50/c52 preamps will meet those requirements.

Integra pre/pros permit bi-amping and they are reasonably priced and great quality overall. The new r1.1 can be had for 1600.00 refurbished and has 11 channels with all recent audio and visual formats supported (with exception of HDMI 2.1 - which isn’t useful yet anyway).  Older models permit biamping as well. 

I think the reasoning behind biamping is that the bass drivers can send artifacts backs to the amp which impact the quality of the higher frequencies thereby degrading soundstage and resolution. As to which is better, I agree that generally a better single amp would make more sense dollar for dollar.

That being said, I'd be interested in playing around with an active crossover :)

There are many technical reasons why active can be better. The key is implementation, as usual.
What you note is an alleged advantage of biamping and in my test it wasn't audible. Active crossovers, on the other hand, have proven sonically superior in my limited experience. And that was not with more expensive amps.

Do you use analog sources or just digital? I focused on digital and designed my system around that, so could take advantage of current technologies for actively crossing over.