Variable phase circuit for sub

Does anyone know if there is a circuit layout available for variable phase adjustment for subwoofer?

I'm planning to build a sub to compliment my stereo speaker setup (Dynaudio BM5 studio monitors) to extend the very lowest frequencies a bit. I would also like to build my own sub amp with active filters and variable phase adjustment. The problem is that I don't have a schematic for this variable phase circuit.....

If anyone can help me I would appreciate it very much

Have a look at Siegfried Linkwitz (of Linkwitz-Riley filter fame) site at:

There is a mass of information on active filters, time alignment, phase alignment etc. If you cannot find what you are looking for here (though you may need to delve into the various project files) then I doubt you will find it anywhere on the web.