do tubes have anything to do with volume?

To simply answer your question -- tubes have nothing to do with volume that I know of.

If you're talking about a lower/higher noise floor -- that is certainly possible.

The thing that baffles me in that you say, "I've got the preamp at about all the way turned up." Just about every active preamp I know of has enough gain to drive any amp into clipping. Why don't you back off the volume a bit on the preamp so the tubed amp doesn't clip when it runs out of steam on on peaks. That is definately not good.

50wpc is not a lot, but without knowing what preamp/amp/speakers you are using, its kind of hard to give good advice.

Of course if your preamp is of the passive level control variety, ignore this.

I think they affect volume. If your tubes are quite old and near the end of their lives, they may sound weaker and definitely less dynamic. Output tubes in my system that are testing at 50% will have muddy bass, soft highs and not a lot of headroom.

how do I test them?

Need a tube tester. Or, find someone in your area with one and bribe/imbibe them. Pittsburgh?