bigal,
I had to make a device similar to yours.I own seven amps for my hometheater.My problem was they are older with no triggers.
I built a box that uses these surplus delay relays.I staggered the turn on times and it now turns on all my amps.One problem was the relays run on 24vdc.I had to use a 24vdc power supply to trigger the big relays.So the first relay is small, it turns on the first amp and the 24vdc power supply and then the whole thing turns on one at a time.All said and done about 100.00 .Plus the ugly box I put it in.