more than four lamps on a 660?

[reefskinner]

Is there any way to run more than four 24" bulbs on a 660? I would like to run six or even eight off of it if I could. The eight by 24" would be equal to the four by 48" in bulb length, but would exceed the wattage a bit. I have wanted to try it, but am a big chicken. If anyone has tried this, did you wire the lamps in a series, or parallel?

TIA........reefskinner