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

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.

Wish I could comment on the Pass Labs gear, as it certainly enjoys a great reputation, but, alas, I have never had it in my system (well, an Aleph 0 back in the 90's!).  I can vouch for the PS Audio BHK 300 mono's, which I have been using for ~2 years now.  They are very nice amps, IME, and they run my TAD CR-1's beautifully.  I think you cannot go wrong with them (and the 6922 tube input for system "tuning" is a very nice feature).  Just my 2 cents.

Pass Labs, no question.

By all accounts Bascom King did a masterful job with the BHK amplifiers. Pass Labs has earned an esteemed reputation. I’d audition both of them at home . I don’t agree that Pass is a no brainer choice compared to the BHK. You must listen and then decide preference. 
Charles