That's a tough one. If you define source very broadly as anything in the chain prior to the speakers, I'd pick source. Specifically, I'd dramatically upgrade cables, analog and pre-amp. I know I'm cheating a bit here, but that's where my system is weakest. The Montana KAS and Sierra Denali amps are very nice and I can live with them for a long while (we'll see). My cabling needs serious help but I'm waiting until I move into my new house and unpack my system before trying new combinations.
So, to answer your question, I would put money into those areas I have not been putting major bucks into before. As many have stated, "more money" does not always equal "better system", but I think it would take some cash to find the right combination and there are several areas of focus: Pre-amp (tube) at $5-$10k, cables (front to back) at $2-$15k, analog (turntable, arm, cartridge, phono-amp) at $10-25k. I'm drooling already. Too bad great analog is so pricey. Oh, and if source includes album, cd and sacd purchases, well -- let's go ahead and add $10-20k there. I'll be happy to let anyone know where they may send a check for $50-70k if you e-mail me. Thanks in advance.