The titans are great, they used to be enough for my college apartment, but I own a home now and the Titans just can't fill the living room. I used to have them connected to a JVC receiver until I bought the Audiosource Amp II and ADCOM GFP-555, I saw an immidiate improvement in sound quality, even more when I bought TARA Labs interconnects.
As far as upgrading the amp to ADCOM, I think I'll stay away from it, I bought (used) the GFP-555 online at a great price, based on ADCOM reputation, and the 555's features but I notice "substantial" noise/static when I added to my system, something which doesn't happen if I send the source (CD) signal directly to the Audiosource amp.
I like NAD, I own their C521 and C-420, but don't think I would buy their products for amp/preamp.
As for the speakers, the Reference 60 and B&W DM603 S2 are very similar (spec wise and $$) but I haven't auditioned them yet.