control from D3 or amps?

[HiFiSi]

It doesn't matter if you use the amp's frequency cutoff or the D3's. It has the same effect.

Using the D3 gives you the convenience of changing the level and cutoff frequency of the sub output from the driver's seat, but only to one of 3 different frequencies. And Using the amp's low pass crossover, if it's the type with the frequency knob, gives you the ability to fine tune the cutoff frequency to exactly where you want it, but you have to go to the amp to do it. There's ups and downs of doing either, so it's basically up to personal preference, really.