Yamaha DTXpress/DTXplorer/DTXtreme

The DTXpress series have always been a little low in headphone volume,
although with the right headphones it is usually ok.  However, if you
can get plenty of volume on a preset song then that is not your problem.

I am not familiar with the DTXpress IV but the other DTX models have a
master volume setting which is worthwhile checking if it has one
(although I would have expected that to affect the preset songs as
well).  There seems to be a headphone volume control on the back of
the DTXpress IV but I don't know if there is another way of adjusting
it as well, through the control knob.

It may be worthwhile picking a single pad (voice) and seeing what the
volume setting is.

The other area where it may be affected is in the trigger edit menu,
although that will be on a pad by pad basis, not overall volume.  

Maybe someone with a DTXpress IV can help with more specific advice.

Keith.