Absolutely no popping when turning on. Very light thump when turning off (that relay doesn't do anything for off) but it's nowhere near as bad as the jarring "POP-tss" I used to get before.

Did have some interference where I could hear my rpm climbing but that was due to low quality 3.5mm cable out of the DAC being close to the rest of the car wiring and disappeared when I moved it.

For the shutting off the relay I linked won't work cause it'll turn off as soon as the power is cut to it. You need power to keep relays closed. It's a 'power on -> wait -> click on relay' deal.

I've used this one (got the same one from ebay for $6. Important that it's the 3 wire FC-30 model) for shut off.

This one has a signal IN wire, and with the correct jumper setting can be constantly powered, click the relay on when the 'IN' gets 12v and delay the relay off click after 'IN' wire loses power.