Seems that subwoofers are a wild cat of sorts. We build monster capable subs and taming them and getting them to behave is the tough part.

I know phase is an issue, but what if your DIY design has no phase adjustment.

Frequency response at the cross over point is a key issue and matching SPL of the mains and center.

Time domain issues. Is it just a delay error for you ?

On any plate amp you will find a phase adjustment.On most all processors you will find a delay settings page.Do these two coincide with one another ?

KG