i have an alpine type R 1221 12inch sub wired in series for 4ohms? I had it powered by a pioneer monoblock 300w amp. I have just replaced the amp with a response 2X150w bridged to 500w rms. I am pretty sure that the sub is 300w RMS, i was reading how these will handle more though. Does anyone know roughly how much the type R will actually handle?

All amps rms output should match the rms of the speaker they will be driving or at least over by
25 %
Example
SUB 500 RMS @ 4 0hm = AMP 500 RMS 4 0hm
2 SUBS 500 RMS 4 0hm =1000 RMS @ 2 ohm = AMP 1000 RMS @ 2 ohm
you need to make sure the subs are the same type and wattage MUST MATCH

With SUBS you need to take into Account that your sub box can change the WATTAGE of you speaker as a sealed box with a sub 100w = 100w but the 100w sub in a ported box could need 200w depending on port size and box Litres