Infinity Perfects would do nicely. People are obviously going to reccomend elemental designs to you... Adire doesn't make a 10" driver, so you're okay there..... I don't know what the DD subs take on the power side of the spectrum, but those would get plenty noisy and i do believe they make a 10" driver. And to stay on the cheap end of things, a jbl 600.1 and ESPECIALLY if he doesn't mind having used equipment..... I got a used 300.1 on ebay for 50 bucks, so you can see how cheap you might have that 600.1 for.....
There's nothing wrong with not spending the whole 600..... then he can upgrade his stage too.
P.S. If you wanna go really cheap....... I have a pair of 10" pioneer subs, used, but in mint condition. They are actually really old, the ones with the diamond plate design on the cone, but i'd let em go for 50 bucks
