500w rms bass, moderate? please disregard this persons statement.
and in a smaller car like a mazda3, you definitely do not need to waste room with another sub
i am running a 100rms amp(sony xplod, got it for free, very low quality amp) on a 400w rms kenwood 12'', and some would consider it too loud.
if i were in your situation i would get a high quality 10'' and 100-250wRMS amplifier. i can guarantee if it is setup correctly you will not be disappointed, and will likely have to turn it down.