If deep and tight bass were the be-all and end-all of amplifier performance, then only solid state amps would be in the market (cheap ones can deliver this). But, there is so much more to performance, even in the bass range. I have no idea if tube amps that are not as tight sounding are coloring the sound and adding in distortion, I only know that I like the upper bass performance of many lower-powered tube amps -- bass has a better sense of subtle and varying "tone," it has less of a dry quality and "mechanical" sameness. I hear the same sort of dry and toneless bass with many high-powered tube amps, so it is not necessarily a solid state vs. tube issue. The amps I tend to like probably have a low damping factor, but whether this is the "cause" of the sound I like or whether it is just something that attends the basic designs that I like, I don't know and don't really care.
In short, I don't think damping factor means much at all, and it certainly isn't a high priority consideration, beyond some minimum that affects basic compatibility with a given pair of speakers. I would not worry about damping factor at all if I had genuine 16 ohm speakers. With an 8 ohm speaker, I might be concerned with an amp with a damping factor below 8 (but I would still audition because there is a good chance the combination will still work out). With 4 ohm speakers, i might be a little concerned with something below 15. I've heard SET amps with 4 ohm speaker that sounded terrific, and many of these don't have have damping factors above 4.
