When I had a Triple Recto, I tried a few different units with good luck:
* Line 6 PODxt
* PODxt Live
* Behringer V-Amp2
* Alesis Quadraverb
All 4 of those worked great in the loop, plus with the PODxt's and V-amp2 I always had a backup incase I blew a tube or fuse in the boogie.... Not a tube amp, but I would get me through the night.