We also have a 6 bulb retro and use ATI, GE & Korallen Zucht Figi Purpl. I am running from front to back:
Blue+
Korallen Zucht Figi Purple
Blue+
GE 6500
Blue+
Blue+

I asked the very same question on Reef Central and this bulb combo was recommended also by Grim Reefer on RC. He is the T5 expert. You can go on his T5 thread, ask a question and he will respond. You can see our tanks here:
http://www.maast.org/showthread.php?...133#post804133

However... it is time for us to replace our bulbs.