When I bought the amp my speakers were Tonian Labs Classic 12.1S and a couple of tube combinations I liked best were 1967 vintage Holland 6V6GT output tubes with Psvane CV-181 input driver and Brimar 5Z4GY NOS rectifier, or NOS Zenith 6V6G coke bottle output tubes with the same CV-181 or Raytheon JAN 6SN7GT, and a RCA 5Y3 rectifier.
Later I used the amp with Spatial M3 Triode Masters and tubes I liked best were JJ KT88 or Genalex Gold Lion KT77 output tubes and the CV-181 with a vintage RCA 5U4G rectifier. With the amp I had, the choice of rectifier had a large impact on the sound. That's not true on the more recent amps Dennis is making.
You have a lot more output tubes to try, should be fun. The only others I had were JJ 6V6S and they didn't sound as good as the 6V6's I mentioned above.
There is a forum dedicated to these Dennis Had amps on Audio Aficionado. It's very long but lots of information on tubes others are using.https://www.audioaficionado.org/showthread.php?t=24170&page=488