Yep, you can go directly to the PA using the XLR out on the back of the preamp. You don't need a DI at all. And, yes, using the main outs you could run into a combo. HOWEVER, you may want to read up on various inputs on the amp before you buy. Some have an input that allows you to bypass the preamp on the combo and use the power section as a "slave amp". This would be best because the signal coming off the preamp might be too hot for the regular input on the amp.
If you're gonna be stupid, you gotta be tough. - My Grandmother