Tube amps are generally not happy with most modern speaker systems - they cannot (in general) deliver the current drive needed to feed speakers with huge dips in their impedance curves.
There are a few new commercial designs that appear to be well supported - Manley is one. I'd be inclined to go for a new or near new amp, most are modern interpretations of classic designs anyway. Old transformers etc are trouble looking for a place to happen.
Or you can wander over to www.diyaudio.com
and scan through the tube forum if you feel the need to build your own...