My only concern is using a power supply you've
'found'.
There may well be protection on the input of the DTX module in
case of have the wrong polaity, but as your supply had 3 Amps if could have
distoryed that.
Providing you didn't seem burning you may well have got away
with it.
Always RTFM first!
As a DTXpress owner I've never really had a problem with the
volume of either the headphone out, or the line outputs.
All I can suggest is trying a portable stereo with an aux
input, or better still a Hi-Fi amp and use that's aux input (don't go using the
phono input please).
If al else fails... Get a DTXpress III module.
Season greetings to all..
the Sonic Energy Authority - a sound
investment