sounds like an input sensitivity issue to me.like maybe the amps are meant to do full power with 4 volts and the pre-amp only puts out 2 volts.something to that effect.first thing i'd do(you probably already have) is see if there are input trim controls on the amps.next check specs(max output voltage for pre-amp)(then something like 200 watts at 4v input for the amp).someone may come along that can help more than me(at least explain it better). are the amps for pro sound?i remember years ago when i was running sound for a local band and during break i would have to max out gains across the board to get a little volume from a tape deck!
what amps are you using?this could help for a definite answer about using a 990.