There are several discussions of damping factor on this site and elsewhere, if you want to spend some time surfing, searching, and reading. I am far from an expert on the subject, but my understanding is this:
- Damping factor relates to an amplifier's ability to control (start and stop quickly) the movement of a speaker and is expressed as a ratio. The higher the damping factor, the "tighter" the control of the amp over the speaker system. 500:1 would, in most cases, be considered quite high. 5:1 would, in most cases, be considered quite low, but you can find examples of each -- and everywhere in between -- in musical instrument and hi-fi amplification.
- Generally speaking, tube amps have a lower damping factor than solid-state amps, but this can vary widely.
- Damping factor can come into play more around a speaker's resonant frequency. Say this is 50 Hz for a given speaker (will vary with the speaker); an amp with a lower damping factor will let the speaker move more freely at and around 50 Hz, which will give you a natural "free bass" boost in that area -- depending on the system and the circumstances, this might or might not be entirely a good thing.
There is more to it than that -- damping factor is affected by everything in the system, including your speaker cables (shorter runs are generally better than longer), and the interactions between amplifier and speaker can be complex and vary with frequency -- but, in the end, you are playing a musical instrument, not a ratio or a series of equations. Ratios and equations have their value, but even the experts here don't know enough of them to account for everything that goes into making a system musically viable or not -- and this is before adding the subjective element of the player.
Generally speaking, I'd be wary of pairing a big, loose-feeling, ported cab with heavy woofers (high inertia -- harder to start and stop) with a tube amp. There is a reason that sealed cabs (in which the air acts as a spring and helps to control woofer movement) are popular with many players of tube amps. There are, however, plenty of ported and plenty of sealed designs which work well with tube amps. If you play it, you will know.
I hope that helps some -- and I hope that my damping factor comment wasn't too distracting. If you can play together the amp and cab that you are considering, that is best. What you feel and what your ears tell you, then, is much more important than anything I type here. I was thinking more in terms of buying a random cab that you haven't played to go with a tube amp -- or buying a cab that you might have liked with a random solid-state head.
There is more that could be said, but that's all I've got for now.
Good luck!