Yamaha DTXpress/DTXplorer/DTXtreme

Hi to all!
I´m searching for a electri drum, and today i went to a store and 
tested Yamaha DTXPress.
I noticed something strange, the phone volume level was to low, even 
if adjust it to MAX. What happened, can the phone level be higher 
without the use of an amplifier?
Thanks, and sorry my english, i´m portuguese!
Bye

Ricardo Teixeira