Yamaha DTXpress/DTXplorer/DTXtreme

Have you tried the DTXpress straight into the 5.1 sound system?  I
used to run my DTXpress through some Logitech Z2300 speakers amd it
was plenty loud enough (I used the headphone socket on the front of
the DTXpress).  If yours is ok when plugged into the speakers direct
then it is a PC problem, which I am probably not much help with.  

However, it is worthwhile checking your settings for volume and mixing
levels on the PC.  Double click the volume control icon (not single
click) and you should get a lot of sliders to adjust (I get 7 on my
PC).  Turn them all up to full - that usually works for me!

Keith.