If you want something that will last and will end the upgrade cycle, passive is the way to do it. The amplifier is going to break a lot sooner than the driver, it would be very lucky to last 10 years. If you take care of the driver, it could last a lot longer than that. Besides that, with an onboard amp, you are stuck with the features and power of that particular unit, but with passive, you have a lot more freedom to shape the signal and power the subwoofer.
If I had an 18.0, I wouldn't even feed it the maximum power handling, maybe 1 kW, I wouldn't even want to drive it close to xmax. The further you keep your driver from xmax, the cleaner it stays (it will also last longer). Something like the Funk or an LMS Ultra will give you lots of super clean output without even coming close to their maximum linear limits. Beauty is in the details, if you want to see something beautiful, just take a look at the 115 dB sweep of the 18.0 in the THD measurement. From 10 to 20 Hz it barely scratches 10% THD, keep in mind, in that range, it would be outputting around 90 to 100 dB. That is far superior to even the RE XXX, and just a hair better than the vaunted LMS Ultra. I would look at it as the cornerstone for what could eventually be a near perfect bass system.
Edited by shadyJ - 12/1/13 at 8:10pm