amp with gain volume control

Hi, I want to bi-amp without bypassing the crossover in my speakers. Any suggestions? I just purchased a Parasound Halo A-23 for the bottom end. With the gain turned up completely it seems as though the bass is weaker than using my single amp. The amp that I am using is the Coincident Technology Frankenstein II. I just need an amp for the bottom end and I do not want to break the bank.