Ive got a RF 501BD..
 

It puts out 300@4ohms. I have a single 12' sub hooked up to it right now. I was wondering..if I bought another identical sub, how would the system pound overall?

I dont quite understand how this whole thing works. Does it put out the same power for both subs?

Thanks.

What sub do you have? Is it Dual voice coil?

Its a infinity kappa perfect 12.1, I think its single voice coil.

Single coil subs are what you are going to need on this amp, a maximum of two for your application. Make sure that the second sub is identical (ideally). The amplifier will make a minimum of 500w rms, split evenly between the two subs. This amp was known for extra power, most coming in close to the 700w rms mark with two subs hooked up, so 350w rms each.

I read somewere that this thing puts out extra power. How would I test exactly how much power it puts out?

Also..If I were to get completely new subs..what would you guys recommend for my application?

If you were looking to get new subs, get two subs that are each 4 ohm single voice coil and rated around 250 - 300 W rms.

Or you could do two double 2 ohm voice coils and wire each sub up to a 4 ohm load then wire the subs in parallel to your amp (which would present a 2 ohm load).

For two subs that would match power wise (according the amp being slightly under rated) I would recommend Alpine SWS-1023D
10" 900W Dual 2-Ohm subwoofer. They are each 300W RMS and wired to a 2 ohm load total, your amp should power them nicely.