I would upgrade the speakers. The Thiels need more power/current to perform at there best. IMO the 1.5's don't justify spending more money on an amp/pre or even a larger Bryston intergrated. Since you like the Thiel sound think outside the box(lol)and consider a Martin Logan-ElectroMotion.
Not a bad idea...however, if he's content with the Thiels, I would suggest auditioning (not buying) a larger, more powerful amp.