Yamaha DTXpress/DTXplorer/DTXtreme

Hi, I just bought a Behringer b300 powered speaker for use with my 
dtxpress1. I spoke to a rep at behringer and they said I should use 
the line input on the speaker instead of the mic input. When I use 
the mic input I get great sound but if I switch it to line the sound 
volume goes way down. He mentioned that the line output of my drum 
module my be to low (any body know what it is... (he mentioned -10db 
on some older ones and then +4db on some newer ones)
and that I should use a DI box to convert the signal to a mic level 
so I could then use the mic input side which has a built in pre amp.
Anybody used this speaker or know what I should do.
Would the fact that I am using a mono cable have any effect?
Should I use a y-cable and combine l-r to a TRS cable then to speaker?

Thanks.