well do you have any friends who may have an xtra 600 you can borrow for an hour or two, or maybe you can take the whole down to the local hydro shop.
How cold is your room? are there already plants in there, cause freeezing doesn't sound so good.

I personally wouldn't put a 400bulb in a 600 ballast.


If you can get ahold of another bulb, plug it in, if the bulb lights up fine, you know the bulb was the problem, if the bulb doesnt light up, you know the ballast is the problem.