I can chip in that there is quite a difference, to my ears. I had it original for a month or two, than upgraded the output capacitors as my system was a bit lean, and this got that back. I also ordered the upgrade as I do like the system much more with this pre than without.
To me this upped the sound overall, like when putting in some piece of gear that is better overall then the last (faster, details, more oumph). As
I am always fiddling with components and diy speakers, and do a lot of measurements to set up crossover and stuff, I am pretty sure the upgrade actually is quite measurable in low end. (Not sure if this is the only reason but my sealed Altec/GPA now get to 30 instead of 40 Hz with the same eq) . For me this was fantastic !
I dont think there is much to worry about, being less tubey, in a way it sounds more linear to me, but on a higher level. Still tubey in the most positive meaning. Do it

/Henrik