I have had subs for a good 4 months now and 2 weeks ago my first set blew so I bought a new set of 10 inch subs. 2 days later the died. I then replaced the box, wires, and amp. The new amp is 300w rms and the subs are 300 w rms in a porter single box. I had the amp set at 3 out of 10 gain and frequency set at 100. I smelt the burning smell and later today sure enough the sub died. What is causing the burning? I know it's not over powered and it's wired in standard connection at 4 ohm.