Anything with a strong power supply. Everyone is suggesting along the same lines- large stiffening caps, lots of output devices and a good sized toroidal transformer. The Coda line is what I would take IF if didn't want gobs of power on tap for lower efficiency monitors.
You have what you need for a tube amp, splurge on some SS and see how it goes. You really can't lose on an S5.5 if you don't like it sell it and you will know better what it is you would like a little cost on the S5.5