Yamaha DTXpress/DTXplorer/DTXtreme

Sorted yet?

Well, I've been experimenting myself just tonight here, and I 
stumble upon something that made me think of your problem...Likely 
it's not the same thing, but I was fiddling around with my 'To Host 
Switch' on the back of my DTXpresss III module & I got some 
strangeness. Changing the switch to the 'wrong' setting made the 
midi drum file played from Cubase into the DTX module activate/turn 
on an internal midi Yamaha song. 
This happends when I hit the play button on my transport bar. When I 
have the correct setting for the To Host switch (PC-2) it dosn't do 
that.

Just mentioning that again, in case you've bumped that rear switch 
to a wrong posistion...?

I've been experimenting with midi recording with my DTXpress kit 
into Cubase and having a software drum sampler play the sounds 
instead of the module. I can load various high quality samples into 
the program for a nearly limitless open ended  system. Though I like 
some sounds within my DTXpress modules, as you do, the ability to 
play sounds from 'In The Box' for midi play back is a very cool 
option.
This is not your question, but just thought I'd mention that.

Hey but let us know what's the fix for future reference.

Steve