Problem solved... I decided to revert to a full range, passive setup, mute the top end, and listen to just the bass. Everything was as it should be; no muddiness etc. So, the bass bin not being the problem, nor the passive network - must be the active crossover... and then it struck me; the Marchand XM-44 is a four way xover: low, low/mid, high/mid and high. I had the 400 low pass modules in the "low" spot when they should have been in the "low/mid" one. I changed them, and voila! Thanks to everyone for the suggestions, I do appreciate that. I think I'll stick with the "hybrid" xover network (active on SS for the bass; passive SET on the top) but I'm wondering about the xover point for the active. The 15C is rated at 63 to 2800 Hz, however we all know that the response is all about the horn. In the LS, the passive (for the K-33) was set at 400 Hz (1977), however that is a bit low for the EV SM120A horn, which is stated to have a "useable" response down to 500 Hz, although in the literature it is given as 800 Hz... (the later Sentry was said to have crossed it at 400...). I'm thinking I need to cross at least at 500 but perhaps higher if the LS bass bin would allow that. Ideally I would go as high as 800 Hz but that's probably unrealistic. Anyone know how high the LS bass bin will go?