Hi, sorry for the confusion about my 4 amp figure 
It is neither peak or RMS, it is instantaneous current at 0V out (which is the same as peak current into shorted load for this type of limiter). 4A at 0V out means there is a great risk of protection activating if peak current at peak output voltage exceeds 8A. 8A peak gives 3 ohms [DCR] * 8A = 24V peak - which is 24*24/(2*4) = 72W in 4 ohms.
If the limiter is single-slope above 0V like this one is then at least half of the peak current should be available around 0V out, this can be seen if all possible load lines for a speaker (which is the DCR in series with an impedance) are plotted.
A signal can be constructed to trigger a protection circuit though, even if this current delivery is met by the amplifier, but I doubt such a signal occurs in music. Especially in a subwoofer amp it seems highly unlikely.
An alternative as you are using lateral MOSFET:s is to use gate clamping zeners set for an appropriate peak output current instead and use fuses for long term protection. The decreasing drain current with increasing temperature make these transistors very robust.
It is neither peak or RMS, it is instantaneous current at 0V out (which is the same as peak current into shorted load for this type of limiter). 4A at 0V out means there is a great risk of protection activating if peak current at peak output voltage exceeds 8A. 8A peak gives 3 ohms [DCR] * 8A = 24V peak - which is 24*24/(2*4) = 72W in 4 ohms.
If the limiter is single-slope above 0V like this one is then at least half of the peak current should be available around 0V out, this can be seen if all possible load lines for a speaker (which is the DCR in series with an impedance) are plotted.
A signal can be constructed to trigger a protection circuit though, even if this current delivery is met by the amplifier, but I doubt such a signal occurs in music. Especially in a subwoofer amp it seems highly unlikely.
An alternative as you are using lateral MOSFET:s is to use gate clamping zeners set for an appropriate peak output current instead and use fuses for long term protection. The decreasing drain current with increasing temperature make these transistors very robust.
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