Starting the tube rolling adventure.
I think the stock 6ZH1P provided lots of low end bass, which bled a little bit into the mids. The bass and the low mids sometimes were a bit bloated/muddy. Just a bit, but it bothered me sometimes. The treble was harsh and piercing/sibilant enough, that sometimes I turned the volume down a bit. The stock tubes also seem noisy. I hear random static noise artifacts. I do listen at higher volumes than most people.
I like the ZDT Jr, but I did expect to swap tubes and explore all those other tubes out there.
ATM, I've installed Western Electric 6AK5 NOS vintage tubes. Less than ten hours in on these tubes, so they are probably not all the way burned in. The WE 6AK5 provides a much improved sound over the stock tubes. There is slightly less bass thump. The bass is equally extended, but more detailed and less one note thumpy. Male vocals are clean and clear. Gone are the bloated and muddy low mids. Upper mids and female vocals are also clearer and detailed. Its easier to hear the different voices in a KPop girl group (LOLS). With the stock tubes, the voices kind of blended together and they were hard to tell apart. The treble is a bit more extended, more detailed, without harshness or sibilance. I can listen at really high volume level without feeling that my ears are getting stabbed with a forks.
Quickly, I think sound stage is wider, maybe deeper. Imaging is pretty decent, but my favorite music is not so demanding in that way.
There are less of those random noise artifacts. I am thinking that rolling the 6N24P might be more helpful in that regard. It does seem to be harder to find 6N24P equivalents. I did buy some more of those Russian NOS tubes, just to have backups in storage.
Listening to a pair of Massdrop X Sennheiser HD 6XX headphones, on the Hi Z output, the volume knob was sometimes over at the 2-3 PM position for some songs. The 6XX became exciting and dynamic. There is some decent bass and the treble is more lively and more extended. I had always felt the 6XX was rolled off and were just a dark, dark headphone. I guess those comments about 6XX being hard to drive are very true.
My recently arrived EMU Teak, (low Z output) sound really good with the ZDT Jr. I usually have them on 9-10am on the volume dial. But I find that the Fostex family (TH-X00 Purpleheart, Ebony, Emu Teak) just sound good with everything.
Overall, the ZDT Jr has a very energetic and dynamic presentation with the WE tubes.
All in all I would definitely recommend rolling a pair WE 6AK5.