SSP45N20A

I've never tried them in the amp you have but the older amps that used that transistor went to a irfb42n20d. The original amps that uses the sss/ssp had no individual gate resistors. The ones that used the irfb42n20d had unequal gate resistors. The low value was 4.75 ohms. The higher value was 39.2 ohms. If you try the irfb42n20d, try it with the gate resistors you have. If the amp does strange things, go to the lower values.