Cross-over upgrade over subwoofer's?

How good are the cross-overs in subs such as martin logan's depth?
Are there outboard units available? Maybe tube versions?
Or should the existing one in the sub be modified?
I have tried different ways of integrating sub.
At this point I am using sub with line level high pass output sent to main amp above 40 hz.
A friend has ML SL3s driven by Parasound JC-1s and a ML Depth. He was using the high-pass filter (70Hz) in the sub and was happy with it. The negative was that he could not use balanced lines to the amps. He switched to an NHT X2 filter and thought it made an obvious improvement. I believe he's using the highest crossover point. He recently added a Velodyne SMS-1 and is now convinced what he's always read about the room being so important.

The take home message is that with an internal crossover you can't utilize EQ on the sub channel. So an external bass management controller like the NHT X2 is likely better than what's built into many subs, but it also allows EQ to be applied.
Tact Audio.