Nothing wrong with a recommendation . Especially since most of the Polk speakers I've heard sound pretty decent, even when given power from an aftermarket head unit (similar RMS rating to the Mach amp). In the end, it's probably best if the end-user auditions some speakers, and lets THEIR ears decide what sounds best.
If it's any consolation, I DO have the money, and I wouldn't go with JL coaxials. Too 'bright' for my taste. Some people swear by them. Just not me.
Check out the Sundown e8v.2 if you want low-budget bass, with a high-dollar sound. 200w rms handling capabilities (saves $$$ on amplification), and in a ported enclosure, it will rock your world. I tried the e8v.1 in my Taurus, in a crappy enclosure. It was sufficiently horrifying right around the port frequency. 250w rms JL 250/1 amp driving it. It could certainly keep up with a mach-amplified set of stock speakers. It kept up with my Boston front components getting 4x the power he'll be sending his speakers.

The infinity reference 8" sub has some good response as well, and those things are DIRT cheap. If you go with a small sub, do a ported enclosure. You will not regret that decision.
Like Ben said, a LOC from the rear speakers would work great (if using the mach amp). I'm not sure what kind of signal the JBL rcu sends to the sub/amp. If it's high-level, you can use the LOC. If it's low-level, you frankenstein-ing some RCA connectors off of it. It will likely be a mono signal.