+ 1 to Doug's Tubes. Tell him the kind of music you play and the tone you aspire to and he can fix you right up. He retubed my Mark IV last year and hit the nail on the head for the tone I was looking for.
KCA Tubes is superb! Mike has a great selection, really knows tubes and amps, and really stands behind his stuff. He's an avid poster at TGP forum, as well, and shares his experience freely. A very satisfied customer here