Hortilux and Ushio. I believe Philips also makes good 600w bulbs. They are a bit expensive, but quality always is. Never use a dimmable ballast (shortens lifespan considerably) and keep the bulb clean of dust and oil (hands) by periodic cleaning with a lint free shammy.

I have a dimmable ballast, but Im not using the dimming options. Does that still shorten the life?

I ended up buying a digilux bulb. I did look at hortilux, and they were definitely expensive. All I could find with Ushio were conversion bulbs for HPS ballasts, or something like that. I didnt know if they would work with mine, which is an ipower ballast.

After 7 days with the new bulb, Im impressed by it. Very bright, great spectrum, and the plants are responding very well to it.

It will be fine, there is nothing wrong with the ballast, just firing a specific watt bulb at varying watts. Different spectrum, waste of energy.

The only time I would use this function is with a bulb rated to a dimmed option. ie. 600w bulb to 600w dimmed from 1000w.

Hortilux is the best IMHO. It is also US Made. I like the bulbs made in Europe but stay away from the Chinese bulbs.

Horilux bulbs can be had with deep discounts on ebay.

I pay 65 bucks for a 1k

Made in the USA... becoming more & more of a rarity in our modern times.