You don't need a 2 ch.
For ~1200w worth of subwoofer power a class D mono would be the obvious choice.

What brand/model is your current amp?
SA's all have dual voice coils. A dual 2 can be ran at either 1 ohm or 4. So at this point it could simply be that you're not getting near as much power to it as you think.

The enclosure is also a major factor in subwoofer performance. The wrong type or size of box can greatly limit a sub's potential.

As for an amp for a pair of SA-12d2's. $150's a tight budget. Basically, you're looking for as much power as you can get at 2 ohms so your options are going to be limited. (You'd be better off with D4's. That would give you a 1 ohm wiring option, where most of the low cost amps make the most power.)

This amp is over-rated for sure, but might give you around 1kw at 2 ohms: http://www.sonicelectronix.com/item_6581...

This is very over-rated, but actual power should be decent: http://www.sonicelectronix.com/item_8918...

better: http://www.sonicelectronix.com/item_7649...

It's not just the amp, but the enclosure that will have more effect on how your subs perform. I had my Sundown SA-12 on the end of a Pioneer amp @ 140 watts, and believe me that was plenty loud enough, because i had designed the enclosure as such.

As for your questions, without knowing your budget, it's hard to recommend a specific amp.

I'll ignore the watt ratings, because the published power ratings from those two manufacturers are usually inflated (peak or RMS).

The impedance (ohms) is the issue. When you combine two 4-ohm subs in parallel, you end up with a 2-ohm load. When you bridge a 2-ohm stable 2-channel amp down to one channel, you can't can't connect it to a 2-ohm load. The minimum load impedance for the bridged amplifier is 4 ohms. So while you could bridge the amp to drive a single sub, with two 4-ohm single voice coil subs your only option is to connect one sub to each channel, making do with the 4-ohm rated power.

car audio guys pleasehelp Im buying new subs today and I cant decide between 2000w dvc targa xxx series or 4500w dvc starsound competiotion?

i have a 2000 watt boss sub i know it wont be running even close to what it says with power and im getting a 2000 watt boss amp also im?

wondering what gauge wiring i should use and also it shouldnt run enough power that ill need a capacitor will it?