Is anyone aware of a 2 channel preamp....

Outlaw Audio RR2160 Stereo Receiver

Not a preamp exclusively, but appears to have the features you are looking for, albeit in a 'receiver':

https://www.outlawaudio.com/products/rr2160.html

So you know, the 2 sub out is usually just a convenience. It is not usually electrically superior to using a Y-splitter.

Parasound has the P5, P7 (recommended) and now the NewClassic 200

http://www.parasound.com/200_Pre.php

I also think the Zpre has a dedicated HT bypass input.

Best,
E

Forgot to mention, read the docs carefully. The P7 has bass management, but in HT mode, it disables it's internal bass management in favor of external.