If you have a SP-8 MKII you might want to hang on to it; it is an excellent preamp; I don't know what you would get for the same $ that would be more pleasing. As for mods, I'd be very careful; you won't know what you have until the parts are changed out - just adding new technology with good specs isn't an assurance of anything except likely different sound. What you have is an all-time classic and I think you ought to leave it as is; Newbee's advice is right on the mark; keep the 8 II and buy whatever you think might be better; once you are sure the new preamp is truly better you can easily sell the ARC - but I'm betting it will be hard to find something considerably better until you spend considerably more. I've heard the SP-3A1, SP-6B, SP-8 MkI (original), SP-8 MkII, SP-10 MkII, and the Ref 2 MkI. You can make a strong case for why the SP-8MkII is the most "livable" of them all (great sound, reasonable price, easy to maintain).
upgrade ARC SP-8
I am running ARC Classic 60's amp, new caps tubes and resister upgrade; with ARC SP-8II pre; Martin Logan CLS-1's; and ML Depth sub. I am looking to upgrade the pre. I am worried about mixing newer tech with an older system, If I spend the bucks will I do not get the bang. Any suggestions on what to look at?