I am sorry Doug. I am new here. Tim I don't even know what a sonic maximizer is. I plan to use the crossover only until I learn more. So should I go with the external or the built in??
The "sonic maximizer" is a feature of the crossover you own. BBE does not document how it really works (lots of pseudo-tehno).
My experience and the experiences of others regarding these types of "maximizer" or "enhancer" processing/processors has been that they are, for the most part, snake oil. Without a technical examination of exactly what this one does (and how it does it), I'd not use it. Again, based on the presence of the sonic maximizer.
Based on your original post, I'm not sure what you're trying to accomplish. Is the 6000 too much for the speakers? Sure, if you run it balls to the walls, full tilt boogie, clip lights flashing or worse, staying on... but under those condition a 3000 could do the same damage. I power a single HF driver in a monitor with a channel of I-Tech 8000.. it's all in how you operate the system.
Whether or not you use the crossover in the sub depends on what you're using for the top box. Whether or not you use external active crossovers or the processing inside the amp depends on other things about your system.