Based on the recommendation from GJcooke, I purchased recently the 6H23 tubes from Upscale Audio. The results have been nothing less than spectacular. All musical parameters have been enhanced by this tube upgrade. The LS25 sounds like a brand new design with these tubes. I suggest that LS25 owners should try a tube upgrade before deciding to upgrade to the MKII version. In fact, I just ordered another set of 6H23 from Upscale, and have absolutely no intentions of ever "upgrading" to the MKII. Please note that the 6H23s are in fact labeled "Sovtek 6922" and appear to be identical to the ARC stock Sovtek 6922s. But beleive me, they sound substantially different: more robust,focused and powerful bass; better refinement; sweeter highs and midrange; larger soundstage with better layering and image focus. And to top it all, enhanced coherence and musicality. PHEW!!!!!!!