Ok. This may be a bit long winded but here it goes. Maybe 5 years ago I bought some Dynaudio Contour 1.8 speakers. The person had told me he modified the crossovers, or improved them. I would say the speakers never sounded quite right. It is possible that I was under powering them but they never sounded very exciting. Well I should say they didn't sound exciting at a low volume. They seem to love being played loud. I moved around a bit and they went from the living room into a room where I was using them as near field monitors. Yes I know probably not a good idea... Anyways I noticed that they did not go very low. I would say about 65 hz then they just rolled off. Which makes sense for a 6.5 driver. I tried different amps and tonight I just said that's it... I took one of the mid bass drivers out of the speakers. And now as a two way they sound great. Almost like a giant contour 1.3. It would seem that taking one of the woofers out would change the ohms from 4 to 8 on the woofer side, and the volume should drop. I actually just turned the treble down to flat and everything seemed to work. I don't know how to describe it but they always sounded a bit messy and now they are focused and more involving. Now I have a couple of nice drivers to put in the car. Discuss.
Your first suggestion that they sound different when loud is a bit uncertain. Are you compensating power compression or other distortions? Possibly both. Not an ideal situation.
With regards to making them a two way, a lot depends on how they were crossed to begin with.
With regards to making them a two way, a lot depends on how they were crossed to begin with.