Is it ok to use an AC leg as ground?
One side of the AC source is connected to the preamp ground.  If you connect the other side of the AC source to ground in your +/- power supply, you will short the AC source when you connect the grounds together.

If you swap the two AC wires on either the preamp or your +/- supply, so that the same wire from the AC source is connected to both grounds, all should be well - but measure the voltage between the two grounds to be sure!