Personally I would skip the VBSS and just use the Titan's with a beefy amp and DSP. The ported 615 with DSP would be more capable on the low end then the dual 12" magnum titan design I brought to Mikes GTG. You could extend the ports on the titan and push tuning down to 30hz or so and likely have great output down into the mid 20's. The woofer in the titan615LX is actually capable of displacing more air then the PA460 in the VBSS (xmax is nearly twice that of the PA460).
I'd look at using an inuke 6000DSP or AX6240 to get the most out of the titans or perhaps a Crown XTi, DSi or CDi 2000 if you can find a good deal on a used one.