Passive, sorta powered, no plate amps, build your own (better than factory (sure), crossover, no crossover.
Sure glad I bought my Revel where they had that all that impedance, damping, Q etc. figured out.
OK, I'm lost, nevermind.
One more time - if you go with a passive (as a passive - ie taking power from the main amp and routing above the crossover point to the main speakers), depending on efficiency it could be too loud, too soft or whatever. Your only potential to decrease the volume of the sub would be to put a resistor in the line to the sub, a yucky way to go.
If you're going to add an outboard amp to a passive sub, then you have - guess what - an active sub If you're going to do that anyway, then buy one where the manu. has already done the matching of all the characteristics mentioned above, rather than guess.
Why cobble something together and hope and pray that it works when manufacturers already have done the legwork?
The reason I mentioned the REL subs before is that they have a unique way of using high level outputs to combine with your speakers.