Attempting Strobist lighting

[EJC]

I finally got the light stands and umbrellas I ordered on E-Bay. My son wanted a cool shot of him at his drum kit. He wanted his drums to be quite noticeable. I though low key light would suit. My strobes are 3 Vivitar 283 flashes with variable potentiometers replacing the thyristors to give a variable output so I have no idea what f stops the are being set to. The drums are light with shot through umbrella slightly above the drums to camera left at about 1/3 power. My son is lit by a flash with a snoot about 2 feet above him and 45 degrees to his left at ¾ power. The third flash is on the ground behind him pointing up the wall at its lowest power setting to tone down the shadow caused by first flash. The wall is about 1.5 feet behind him. I just adjusted levels, dodged the whites of his eye and burned the glare on the drums. Do you have any tips to improve this shot?

Thanks
EJC