Hi Art,
Do you know if that is +/- 23mm, or +/- 11.5mm for the JBL driver? Either way, that's a lot.... but if the box is tuned correctly for what you're trying to do, I don't really see 8mm of xmax as an issue.
Jeff,
I'd have him build some boxes for the 4018lf. You can get them for about 225 each.. if you need a source PM me. That and $150 of other supplies would be a bare-bones dual 18 sub. Probably be able to build 2 for the price of 1 srx 728 and I'd guess the dual 4018lf cab would perform pretty close, dare I say better than the srx 728.
The two cabs would probably have about the same resale as a single 728, and depending on his woodworking abilities, he may or may not be able to build a cab that works. He can save money if his time is considered free. he could probably sell the cabs for $400/each down the road without too much issue .. so his net cost of ownership would be just $400. That, to me, is a pretty cheap option.