Good to see you here again Tom.
When I got my first LS5, the MK II, I put in RAM Labs 6DJ8 that I had purchased years ago for my SP10. The RAM tubes added a little more warmth and smoothness to the stock LS5 tubes at the time....I am not sure if they were sovteks then....this was early 1995. You might want to contact RAM to see if they still have such tubes. http://www.ramlabs-musicreference.com/index.htm
A few years later I tried the Amperex and these worked well but I do not recall much of an improvement here. I bought these from a private party so perhaps they were beyond their prime. And so when a couple of these died, I ended up buying the Sovteks and the LS5's character was pretty much the same.
Just last year, I was able to audition the MK III against my MK II in my home as a local audiophile was looking to sell his MK III. And we compared to his LS22 also in my system. It was quite interesting to jump between the 3 preamps, all warmed up a couple hours before listening.
As Albert refers to the other tubes murdering the Sovtek, both LS5s did this to the 22, and then I would say the MK III severely wounded the II. The III amazingly enough had all of the II's midrange magic in tact and yet the III had far more resolution in the lower mids, but more importantly, the III had a natural top end. Unfortunately the II has an exaggerated top end with too much sibilance. After listening to the III for awhile, it was truly difficult to return to the II.....and I had owned a II for 5+ years. The II however did seem to have more extension in the bass .... perhaps this too was exaggerated.
So my point to all this is, before you go out and spend a fortune on tubes, you might want to keep your eyes out for a III. I might even go out on a limb and predict the III with the Sovteks would beat the II in any tube configuration. Albert may rightfully so correct me on this, but the sound of both units with Sovteks was similar and yet the III was clearly in a whole new league. I can only imagine what the III might sound like if Albert and Ed are correct about the other tubes. I just might give Ed a ring too!
Happy to see you finally said adios to the LS2. Right on!
John