I've never comfortably recommended 1200 watts of continuous input power, as that is the absolute maximum limit anyone should ever use on the RL-p. Quite honestly, running in the range of 700-900 watts will be much easier on the driver, and the gains you'll get from bumpin' up to 1200 watts from 900 would be negligeable anyway (unless you're a dB freak and compete //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif ). You can use your 1200 watt Nine.1 (as quite a few of my customers do), but you just need to make sure that your gain is set conservatively, otherwise you risk overdriving and damaging the driver.
You seem to be against wanting to change your enclosure, but you should be aware that the enclosure is one of the most important aspects of a subwoofer system - just as important as the driver used, in determining what frequency response and output you will ultimately achieve. I have no doubt that the 0.8 - 0.85 cu ft net volume for the RL-p12 is attenuating the output you could receive if using a sealed enclosure at 1.25 - 1.35 cu ft net. If you refuse to change your enclosure, there is not much else you can do to increase your perceived output except adding polyfill - but that won't make a terribly large difference anyway.
Build a new enclosure man!
Give that RL-p some room to breathe!
.. and if you're really looking for a killer gain in output over your previous 12" driver, get a 1.5 cu ft net vented box tuned to 34 Hz. If you would like any help with possible enclosures, shoot me over another email. I don't think there is much else anyone on these forums will be able to do for you at this point. Quite a few potential options have been laid out for you, now it's time for you to decide which (if any) route(s) to take..
