Great Northern Sound Company is also another good choice for ARC mods. The guy used to work at ARC for something like 20 years. I've gone through LS-1, LS-3, LS-2B Mk II, and now LS-25 Mk I w/ GNSC Ref mod.
I've rolled many tubes in my LS-2B, but the LS-25 was a big improvement over the LS-2B Mk II. As I've not heard a stock LS-25, I am not sure how much of the difference should be attributed to the mod.
I had also done a side-by-side comparsion of my LS-25 with the Ref 2 Mk II. Per the judging "panel", the difference was fairly small between the two. At that time, I had a quad of Siemens 6922s in the LS-25. They are not my favorite tubes in ARC gears. I later swapped them out for some Amperex 7308s and also tried some Amperex 6DJ8s. I think with the swap of tubes, the modded LS-25 could stand shoulder to shoulder against the Ref 2 Mk II.
One thing about mods is that you will never recoup the cost. I had purchased mine from a dealer. The previous owner had done the mod, and decided to trade it in for something else in less than a year (Found that out talking to GNSC). Had I purchased a used stock LS-25 at that time, and send it in to get it modded, I would be spending just about the same amount as buying a used Ref 2 Mk II.
FrankC