I don't have one, but I'm certain this info from the owner's manual
https://www.decware.com/newsite/SE84TSowners%20manual.pdf is correct:
The SE84TS is designed to use your choice of
three different input (or signal) tubes.
12AX7 - Most Gain
12AT7 - Medium Gain
12AU7 - Low Gain
Each of these tubes will lend a different signature
to the overall sound of the amplifier. If
you are using a standard 2 volt source like a CD
player with NO preamp, you will probably find
the 12AU7 to have not enough gain. With a
preamp, it could become your favorite tube.
The SE84TS is designed around the 5Y3GT rectifier
tube. It is however, compatible with the
following alternatives - each having a subtle effect
on the overall sound characteristics of the
amp.
◊ 5U4
◊ 5AR4
◊ GZ34
◊ 274B
The different ratings and efficiencies of these
rectifiers will effect the operating voltage points
throughout the amplifier which is why they can
have an effect on the sound. You will find that
even different brands of the same tube will
sound different. This is true with any vacuum
tube.
Hope that helps.