have you tried playing each of the subs on its own 1 at a time cause the L7 has 1000 watts rms and the L5 only has 750 watts rms so the amp might not like the different watts and it might be affecting the subs performance. and to get the most out of your amp there should be 2 ohms final going into it. i would recomend 2, 15" 2ohm L5's wired to 4 ohms each. that's what i have and they sound great with my zx2500.1.