I have no idea what the MEC sounds like and I'm not a fan of Warwick basses in general or any of the sounds I've heard them make in particular, but the OBP3 is a stonking preamp. It has a huge tone, almost too much. A little goes a very long way. Did I say it sounds good? You can run it at 9v or 18v but Aguilar recommend you remove one of the battery clips if you go 9v.
I sold a J5 with the OBP3 preamp and I'd rate it as one of the best basses I've owned. It was sold because I found, in spite of all the good points, I still preferred my passive P bass. People buy and sell gear for a variety of reasons, so I wouldn't draw any conclusions from the "merry-go-round" that is the Basschat classifieds.