I am the proud owner of several older PG amps. I have a ZX500, ZX450, and a ZX475ti. My question is, does the next generation of their titanium amps (500.4ti, 600.1ti, etc) have the same guts in them as the older ZX line? Their specs appear to be the same and aside from a few cosmetic changes, they look nearly identical. But I was wondering if they are the same quality as the older ones. I was looking to buy some used PG amps off eBay and was trying to do a little research first. Anyone?