I have the following problem: I have two amplifiers, one is a mono amp for a subwoofer and the other is a stereo amp for two two-way speaker enclosures.

The subwoofer and two-way enclosures have a good overlap in the bass frequencies, however, due to the low-pass filter in the subwoofer amp, there's a phase shift ranging from 60º to 50º in the 0-50 Hz range, the subwoofer being the one "lagging". 

And as all speakers are almost the same distance to me (and I can't change their positions), this causes the interference to be not completely constructive at the spot where I listen to them.

But all leading current circuits I see are also high-pass filters, which would be a problem for my sub. 

So, how can I shift the phase without causing an dramatic change in frequency response?

Thanks!