Audphile 1,
I had both the earlier LS15 with REL caps and sold it for a BAT 5i (which I never really warmed up to...) about 4 months later, I sold the BAT and returned to the ARC and a later model 15 with the Infinicaps. My system remained the same otherwise.( then Plinius SA 100, VSA VR2's Wadia 23) I thought the sound was more linear from top to bottom. That is..better at the frq. extremes, although more subtle, and not huge in comparison. I gained a little air on top and less grain from my previous older 15 as I recall thinking.I was very pleased with the Infinicap 15's sound.
Just as a side note regarding the BAT...The BAT 5i, although very good sounding and was less grainy overall, more liquid in the mids, yet was more closed in sounding than the ARC.Sort of rolled off on top to me. THe BAt's bass was extrodinary and I did miss that extension and authority in the lower registers. Yet, overall, I thought that, at least in my system, both of the ARC LS15('s) had a more open, natural, and believable sound.
Jafox does have a good point however, and one which would be a very big step up from the 15 and 16 in going to the LS5 of any iteration,( Mk1,II, or III) especially with the GNS mod. That would be a clear winner sonically
If your keeping your 15 and upgrade it through ARC.. you wont recoup the cost of the upgrade should you sell it as Warnerwh rightly points out. and you would probably want to tube roll into something like a quad of Siemens/Amperex early 7308's or Tungsrams red print HM 6922. The Russian 6H23N-EB SLNM Cryo's tubes from ATSI are also very good in the ARC. Happy Listening! :)