Have opportunity to buy BHK 300's or Pass Labs 250.8 for my Thiel 3.7's- thoughts??

All things being equal, my bias would be towards monoblocks, but why take someone else's word for it?   ;-)
PS Audio offers free in-home audition of their products if you contact them.
Likewise, Mark Sammut at Reno HiFi (renohifi.com) can arrange a trial of Pass Labs amps.  In this case, he will pay shipping to you, but you have to pick up the return shipping costs.
I own products from both companies (direct from PS Audio and through Mark in the case of Pass) and both know hot to make customers very happy.

Pass Labs without hesitation.

Forgot to mention that Reno HiFi makes a market in demo and used Pass gear (which include current production models). Considering such could move you up the Pass Labs amplifier food chain to a very significant degree while staying within your budget.

Demo usually means the gear has been unboxed and played for a month or two (you essentially get a pre burned-in amplifier), and these units come at a significant discount with the balance of the factory warranty. Some of the used pieces have been "refurbished" at the factory to make sure they are performing to "as new" specs and come with a 1 - year warranty.

Best of all, Mark will buy back anything he sells you towards future trade-ins. I've been a happy camper customer for over 15 years and have bought and sold my way through 4 Pass amps and 3 preamps with Mark while working up to my current system with XA point 8 monoblocks.