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

Pass amps are in their third generation, designated "point 8," so the stereophile review of the "XA-160" is fairly dated.   Here is a more recent, direct comparison of the Pass XA160.8 and PS Audio BHK 300's by Tony Cordesman:  http://www.theabsolutesound.com/articles/ps-audio-bhk-signature-300-mono-power-amplifier/
Spoiler alert: he keeps the BHK's as a second reference. (Must be nice).
 
Note the Pass X- series operate for the first few watts in class A and then transition to class A-B.  The "XA" amps are single-ended class A throughout their rated operating range.  The two designs have different - but still excellent - sounds (the X series being a bit more "punchy" and the XA's more "tube-like").  If you poke around on the Pass website, there is a technical paper by Nelson where he mentions how customer preferences for the sound of his designs relate to residual distortion products.  Fascinating stuff.

You have a tough choice.  King and Pass are both legendary designers.  Gotta listen for yourself.

Thank you everyone.  Believe it or not BOTH deals fell through one of which was as a result of an unreliable seller- but so be it,

If I may, do you have any thoughts about Quicksilver V4 mono's?   I have always wanted to try tubes with the Thiels.   I will probably contact Mike Sanders in the morning.  

@Unsound- I generally do not listen very loud although sometimes I do crank the music up (80db or so would be my guess for an upper limit) and I sit some 10 feet away from the speakers.

^It’s not always about ultimate volume but rather about operating in the realm of the load of the particular speaker.