Internet direct.
Many good internet direct companies including AV123, Outlaw, Axiom Audio, SVS, HSU, etc. have a 30 day demo/trial period in which you can return the equipment for a full refund. See their sites for specifics but in most cases you are only out the shipping. Some also have good "upgrade" policies.
On the AV123.com forum, they have some comparisions of the UL pre-pro and the more expensive Outlaw 990. I doubt you could hear a difference between amps, so stick with the one that matches.