Tube pre for Forte 4a

Hey guys. Loving my new amp despite the Majik in the loop. Looking forward to swapping it out with something more involving, and would like to play with tubes. So the first question is: how well does this amp mate with tube preamps? Second, any recommendations?

My budget is under $1,500US and if I can tuck it under a grand, then all the better (that means I can get it earlier).

Thanks in advance for the thoughts.