Yamaha DTXpress/DTXplorer/DTXtreme

All I do is search and search on what to do about monitoring. I need 
input on base amps versus keyboard amps. I need input on using an amp 
versus using studio monitors. Also, I have been thinking about 
putting together some type of speaker (build a case and use 6x8 
midrange car speakers and 10"/12" component subwoofers). If anyone 
has built a small system themselves, I could also use input on that. 
Maybe a subwoofer and some sort of mid-range speaker. What do you 
think is right?

P.S. Please reply with what monitoring system you use on your current 
set-up and how it has worked for you thus far...it would be very 
helpful.

-Zack-