Connecting AVIC-z120bt to GM-d8500m amp

[Ub3rd0rk]

I just installed a complete Pioneer system that includes, Avic-z120bt head unit, a GM-d9500f amp powering 2 sets of TS-d1720c components, a GM-d8500m powering 2 TS-sw2501s4 slim subs, a GM-d7500m powering 1 TS-sw841d slim sub. Every thing seemed straight forward untill I was hooking up the GM-d8500m amp. The amp has a non switchable LPF crossover, as does the subwoofer output on the head unit. I would like to be able to switch off the subs from the head unit if needed without shutting off the front or rear channel. So my question is, how should I set up 2 crossovers to work together and not kill the sound? Or should I look for another amp and replace the one I already have? Any other suggestions? I do find it rather strange that these 2 components from the same company don't seem to mesh well, even the cheaper smaller GM-d7500m amp can turn off the crossover.