You could look at the Scanspeak 30W/4558:
It has very good XMAX. It's an aluminum cone and the moving mass is less (less weight). It has a lower resonance frequency than the AD12-300 (17hz vs. 24.6hz of the AD12). It has a higher VAS than the AD12. It is more expensive, but it's a very nice sub.
Couple other options:
https://www.parts-express.com/peerless-by-tymphany-835017-12-aluminum-cone-xxls-subwoofer--264-1114
https://www.parts-express.com/eminence-lab-12c-high-power-12-subwoofer-speaker-driver-4-ohm--290-668
Neither one has as good of XMAX as Scanspeak, but the Eminence has a higher power rating. I would not get too attached to a high power rating. I have some SonicCraft designed 12" woofers that are supposed to be 150 watts RMS, but I have driven them very loud with no problem. You just don't want constant high power bass. For home theater, this is just fine as loud bass is infrequent.