Plankie amp hum

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Tzs503gp

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I’m using a Geco Emma plankie amp to drive a pair of subwoofers. I’ve had this amp connected to a Wadia DAC before, over RCA driving a pair of small monitors, and it was quiet.
Since I’ve connected it to the subs, I’ve got a barely tolerable hum over the subs. Its audible from the listener position, but It doesn’t bother me while playing music ( mostly). It has a big turn on thump, and when turned off ( unplugged as there is no ON/OFF switch) after a few seconds, a huge buzz from the subs which fades gradually.
The toroidal transformer is fed directly from a two core power cable- no earth.
The inputs are fed from XLR to RCA cables. The XLR “cold” is shorted to ground on the XLR connection.

I’ve posted a picture of the plankie here. Can someone give me an idea of what I need to do to get this thing quiet please. Happy to answer any questions.

Additional info: if I turn the active crossover feeding the amp off, I get a massive buzz over the subs, as when one touches the live of an RCA connection.
 

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