MTX subs and amp question?

I have two MTX terminators 1200 watt max 600 RMS. I was running a MA audio 800 Watt Mono and it hit real hard before it started cutting. So I out took it to a dealer that deals in sound equipment and he guaranteed me that the Volfenhag ZX-8100 was the amp that I needed. So I bought it took it home and hooked it up and now I have less base than I had with my first system. What do I do? What amp should I buy to make them hit hard and not blow and not cost an arm and a leg? That being said I already have 5 hundred in this system.