Fitted Sub, centre speaker now crackling

[Shiggs]

Help please.

I have a SAAB 93 running on standard speakers.
I have had a Kenwood DNX5190DABS head unit, 50w x 4, running the system for 2 years with a Kenwood KSCSW11 150w under seat sub running from the RCA outlets on the head unit. The system has run fine with no issues.
This weekend I have removed the KSCSW11 and replaced it with a boot/trunk JUICE A12SL 900w enclosed sub.
I have run new cabling into the battery etc down one side of the car and new RCA cables to the head unit down the other.

I kicked it in and for 10 mins or so all was fine.
Whilst setting it up I switched the Gain from 0 to 180 and back again a couple of times whilst listening to tune the sound and the centre dash mounted speaker started to make a terrible interference type sound.
It makes it as soon as the head unit is powered up and carries on for a few seconds after the head unit is powered down.

The sub and rest of the system appears to run correctly.

Any ideas please?

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