Yamaha DTXpress/DTXplorer/DTXtreme

Hi, I'm a newby here with a brain upgrade question. I currently have 
a DTXII and am thinking about upgrading the brain to a DTXIII. 
Anybody know who sells just the brains? Would the DTXIII be 
compatible with the trigger pads that came with the DTXII kit (TP65, 
PCY65, etc)? And last but most important would the sounds be 
significantly better? I understand the cymbals are supposed to be 
improved. I'm also hoping there are some improvements to the tom 
sounds. I don't seem to be getting a good sharp stick-impact 
punchiness out of the DTXII presets. Any chance that's been improved 
in the DTXIII? Any advice here would be appreciated.