Kranzy has the idea with setup. Or buy a mono.
As for subs not "keeping up" when playing metal, is well, bs, as is a 6x9 giving more punch than a sub, as are sub tubes.
The only time it may seem like it aint keeping up is when you've got it cranked to 11, and probably underpowered, so the cone and voice coil will be flapping away like grandmas tits in a soft breeze. Subs LOVE power, feed them and they will reproduce your low end as god intended - tight, crisp with enough punch to make your kidneys bleed. Generic ported boxes are fail, stick with a sealed job, then find an amp that will output double what your sub is rated at: 300rms rated sub, 600rms amp, just remember you can cut the gain back when you are over powered, when you are underpowered and max everything out on your pissy 100rms 2 channel, something has to give, whether its the amp clipping its ass off, or the sub it poling (whilst sounding like someone kicking an empty 44 gallon drum) its one or the other.
Atm, im running an MTX 12" rated at 300rms, im running a mono that pumps out about 650rms, with about 3/4 gain, then tuned to my ear, and it kicks along like a angry mule to Inflames, Machine Head, Opeth and like, but is subtle enough to make Fleetwood Mac sound how it should, and has been running like that for about 4 years.
In short, you can make a 15" be just as punchy and responsive as an 10", its all about the right box, the right amount of power and the right tuning.
Keep the sub, buy a monoblock amp. Job done