I have a THD Univalve amp (that I don't play much anymore and lean on my Marshall and Orange amps), but...I had a lot of fun collecting rare and exotic tubes for the Univalve. For me, some of the best were 6K6s...like NOS stuff from a long time ago from Visseaux and MWT (Marconi Wireless Telegraph, IIRC). I also have a Tung Sol 5881 that sounds awesome.
Most of my finds were power tubes, but I searched long and wide and spent big bucks (probably less than $75
) on an NOS Mullard ECC83 preamp tube. I probably have 40-50 tubes and most are NOS stuff from a long time ago. Fun hobby...