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The amp.make is a Jet Sound, hence the JS in the js4090p model name. The earth connection is a constant earth although I haven't tested the resistance. I have fitted a switch to the remote wire which should more than cope with the voltage from the head unit. If the switch is turned off for a few seconds and on again, the amp and sub will jump back into life. Had a ride home from work tonight for 35 mins with no problem at all. I think the concern lies within the amp rather than good supplies and earths etc.
I was concidering getting a Sony CDX-L410 because it has a good bass equaliser for getting good sound from my 6x9's. Getting back to the earth connection, I've used a Halfords 600 watt Amp kit, and fitted the earth lead as in the kit... but i feel the earth isn't the problem as I've explained above. The head unit fitted at the moment is a JVC mask something or other. This has two pairs of pre amp outputs. I am using the rear outputs (2) to send a supply to the amp.