Have you spoken to the artist you're booking before the agency?, they'll likely tell you who their prefer agent is. I've heard and seen first hand the horror stories of booking an artist through an agency that say they can get that person only for the artist to etiher not be booked for the date or the agency haven't checked with the artist if that booking clashes and after that getting the cash back is near impossible.

Personally I wouldn't deal with agencies, but then I never bothered with booking big names. Then again I rarely did a night which took money on the door, since it's a f. nightmare.

Trouble is the big name artists are stuck having to accept work through an agency in order to get more work from them, a kind of forced exclusivity.