Blair - If you have an amp shutting down you probably have a damaged amp or a damaged subwoofer that is shorting the amplifier out and shutting it down. Even if neither of those situations where happening it still makes case for upgrading out of stock performance. What that upgrade is and how much your willing to spend on performance are going to be entirely dependent upon your budget. The SX12 / 12D are good solid woofers. When paired with the SM600.1 amplifier and mounted in a proper enclosure I'd think you'd see a night and day difference from where you are at currently.
There are 3 levels of performance within the Exile lineup. I'd characterize them like this:
SX12 - a big step beyond stock
XI12 - Uur enthusiast woofer. Bigger voice coils, unique laminate cone, increased magnet, also ships with a grill. Handles more power.
Big12 - King of the lake. This woofer will hurt ya. Aimed at a narrow part of the consumer market that wants MAX output and willing to pair with very big mono blocks.
Hope this helps.