Direct Heated Triodes - no - never owned an amp or pre that used them. I did say "I canna speak to SET world tubes." No doubt there are exceptions.
Can you offer an example of a fundamental change in sonic characteristics that comes from changing tubes?
Character is a broad term. Changing tubes changes drive, balance (bass/treble), transparency/muddiness, and dynamics. If you compare Amp or phono A to B, where B is fixed, tube rolling A can change your preference whether you like it less or more than B.
With Allnic 3000, I found it less than the half priced Zesto. I mentioned this on Myles' forum and he said I should have swapped the recti. Later, at UK Paul's, I listened to his logans with the stock allnic recti and it lack EE and drive. I swapped the recti and loved the system. After that, he continued to use that (KR recti) and upgraded that recti too. Using KR recti or We recti is quite common among Allnic users. I then went back to the previous place of compare of Zesto, and found that swapping recti not only made me prefer Allnic to the Zesto, but also to a phono priced almost twice (the name escapes me, it is the Greek phono that Kronos uses in a two box for their phono)
In fact I don't pay any attention to Allnic reports with the stock recti. Check the forum you will find all happy Allnic users having changed recti. Allnic gear in stock is colored pretty and sluggish and changing valves makes it more neutral and dynamic
This is the write up on Pal's visit
https://www.whatsbestforum.com/threads/the-marvellous-martin-logan-clx-art-–-is-definitely-a-work-of-“art”.21449/post-472845
In another system, we preferred Kiseki to Lyra Atlas and vdh strad when the amp had lesser (muddier, not transparent) 300bs. Next day after changing amp tubes to KR and Elrog (much more transparency, changes to recordings and upstream gear showing through), we preferred Lyra and VDH while kiseki sounded muddied. So, the compromised amp tubes had muddied up everything upstream. We were able to repeat this process, i.e. put the lesser valves back in and make the kiseki sound better.
801a valves - loved the General's system when he had cunninghams in the Mayer pre, as compared to Elrog. Later, I read a shootout in 6 moons with three of the writers and Mayer which confirmed they preferred the other valves to Elrog.
Lampi - there are like thousands of posts across forums. It is pointless trying to assess Lampi without tube rolling. Also, rolling tube changes the gain of the system, as here different valves have different specs and output
211s - quite common to find older NOS Amperex and United and WE tubes transforming. Again, I think it is pointless to assess any such components unless you have the few recommended tubes to try out with it. You will likely reach an incorrect conclusion
Aesthetix IO eclipse reports are the same though I personally have experience tube rolling there. But again I would not go with a report with stock valves
So yes, changing tubes changes what you will conclude about the component as well as the system. Preference will play a role in the conclusion, but midrange, muddiness, bass, transparency, overall balance are all affected.
Noise floor can change drastically with tubes too. You will be able to hear much inflections and contrasts with the quieter ones.