Audio signal sensing auto on/off switch

If the sources have 12V trigger output, you can use a relay controlled outlet for your subwoofer amp.

https://www.adafruit.com/product/2935?gclid=EAIaIQobChMI8Zmxyvum_AIVgBXUAR1UAAuGEAQYAiABEgJM7vD_BwE

The Tricklestar strip is an excellent suggestion, it's a current sensing device. 

If you're looking for a ready to use 'box' audio signal sensing switch, check out the RDL ST-ACR1

On one receiver, I have a 12V trigger. Is that 12V linked to ’power on/off’ of the receiver? How many mA can those 12V triggers typically produce (to directly control a relay)?

Receiver 12V trigger output usually can provide up to 50-100mA. If you want to directly control a relay, my suggestion is utilize solid state relay since its input is optocouplered and only require a few mA to activate. To control 1500W power amplifier, choose a SSR can handle 40A or higher.

Solid State Relay

Will now try if the big phat 1500W power amp can be controlled (I assume it has a ’softstart’ to not smoke that power strip right a way.

If your ’fancy’ power strip ’controlled’ outlets can’t handle the load of 1500W amplifier for long run, I suggest diy a box with 120V power relay/contactor controlled outlet plug into the power strip.

120V Power Relay

AC-AC SSR

I've never seen  a source with a trigger in that price range. And who would want a one source system?

@fuzztone 

I meant the source to provide audio signal to the subwoofer amp(s), it can be a integrated amp, receiver or pre/pro.

Is that 12V linked to 'power on/off' of the receiver?

@kraftwerkturbo 

The 12V trigger output will be OFF if the receiver @ standby, it will turn on went the receiver power up.

BTW, if you choose not to diy, the relay controlled outlet box that I suggested on the first post can be controlled by Universal control voltage 3-60VDC or 12-120VAC, or connect directly to a 12VDC trigger, or even 120VAC.

Sure this box can use 12DV trigger, not just a digital input?

Yes, I can confirm that this box can be controlled with 12VDC. I have one, what you need to do is connect the green plug to the 12V trigger output.