No mods. done, not necessary IMHO. But have done extensive tube rolling just for the fun of it! Among the very best were Tesla ECC803S - neutral, airy and hard to beat. Various 5751's tried including GE, Raytheon windmill. Position V3 is now occupied by a 5751, proving more focused, less vague than any 12AX7. Positions V1 and V2, however, one particular 12AX7 won me over in no uncertain terms - get them if you can find them - MAZDA 12AX7 GREY PLATE 1957, it's as if these were made for the job, just AWESOME!