Bryston are wonderful amps; incredibly well built.
The warranty is 20 years, parts and labour, transferrable. You pay shipping one-way. Yours is obviously out of warranty but Brystons are still some of the best built and most reliable amps you'll find.
The 3B is only rated at 150wpc if I remember, so driving two 18" bins sounds a bit optimistic! Try it on your fullrange speakers and I'm sure you'll be happy.