I am assuming that you already know the advantages of solid state, i.e. size, reliability, longevity, etc. So these links might round out your knowledge base and help show some merits in the old school.

Some say valves just sound better, in spite of the increased low-order distortion. Some claim that they have less high-order distortion, and there is little doubt that they clip smoother, with rounded corners at saturation, so there is some merit in that. And another thing that's kinda funny is that I'm told they aren't damaged by EMP, so they'll survive a nuclear blast or UFO.

Seriously, I own both solid state and tube amps and they both are nice. The glow of tubes is nostalgic. They're mesmerizing like a fireplace, and hi-fi tube amps don't sound like you might expect. It isn't like the 1940's public address sound that people unfamiliar with good tube hi-fi might imagine. They sound very nice.