Powering up/down sequence with a 12v dc trigger

Are you using some kind of ac power conditioner or something with relays and delays? Your use of 12v trigger indicates this...
1.Amplifiers, on last and off first.
2.If you have an active crossover or something like a DSP between the preamp and power amps; on before preamp and power amp. Off, depends on answer to #2.
3.Preamp, on second (if you have #2), off second or at the same time as #2. 
By "source", what do you mean? DAC, CD player, turntable, tuner, streamer or what?
Are we talking, SS, tube, class D? Please provide MORE DETAILS before following my advice above!

Macs can be assigned and sequenced. They also have a programmable delay.. Up and down.. Would that help?

I still boot by hand on the newer stuff. :-)

I still use a Variac to bring up my MC60s and 225-275 valve amps.. A 20 minute process. 25 vac to 50 to 75 to 90 to 105 or so..

Triggers.. are fancy.. good for HT and multi HD amps coming online..

The pre is in standby, streamer is left on and class ds are left on in the summer.. no need for triggers here.
Turn on the phono/tape pre (at least an hour before use). No trigger there.. No HT, no need..

A lot of NOs LOL.. grandchild... gear.. same thing...

Regards

Good question.  I assume pre has to provide for that.  My pre (Dac) never fully turns off since it operates with remote. It provides proper sequence for the power amp.