For the brands you mentioned, the order that I would recommend would be probably JBL, Pioneer, Kenwood. Kenwood is only last because I have not heard one. The Vifas/MS 2601s would be a substantial improvement over any of the previosly mentioned subs.
If I were you, I would get a 2 channel amp and run it off of the RCAs on the HU. The kenwood may even have a sub output. If it does, you may not even need a crossover if the HU has one or If the amp has one. If your HU does not have a sub out, definitely get an amp with built in crossover.
