Verified this afternoon that the Sovtek LPS + LPS + Mesa 12AX7A (V1+V2+V3) combo sounds flat out wonderful in my 900 through a GS112 at volume. I wasn't able to spend a ton of time with it, but first impression is that the tone controls seem to behave differently, a bit less extreme, more relaxed, would be my best description. Almost like the Q has been changed - possibly widened - or maybe the gain structure changed/reduced? As a reference, with the stock tubes, I was having to pull back significantly at 225Hz - to about 7 o'clock - to try to clean up the "mud" of an overpowering low-mid push I was getting through my GS cabs. With the LPS/Mesa combo, not only did I find myself comfortably able to set the 225Hz higher than before, but I was finally able to use some of the 600Hz and 2.5Kz as well, as they were no longer over-powering the tone. This is not a massive change, but significant enough that I can hear/feel it, and seems to have reawakened my hope for the STL900. It would appear I now have a bit more sonic flexibility than before, and to be honest, this is as close as the 900 has ever sounded to emulating some of the things I love about my Walkabout. I am left with a massive grin on my face. I have a gig on Saturday to check the LPS/Mesa tube combo out. Should be interesting to see if what I'm hearing translates to the stage. I'll also keep this a secret from our sound man and see if he's able to notice a difference or not.
Oh... the stock Ruby in V3 also sounded very good with the LPS in V1 and V2. I'd say a bit more high-end "air" and a tad more aggressive sounding than the Mesa 12AX7A, but still better by far than the stock setup. Based on this experience with my Sovtek LPS tubes, I am putting in an order for a 5751 today to see what that does. I'm guessing I'm going to love a 5751 + LPS + Mesa or Ruby 12AX7A combo.
All this IMO of course, so your results may vary.