I'm very happy using Tung-Sol 2C51s on the Valhalla 2--a little warmer than stock, a bit more bass, nice soundstage. Using them with HD 6XX and HD700 (not popular here to put it mildly, but I like them.) FWIW, Mike Moffat has posted that he prefers this tube family (2C51, 5670, WE396A) to any 6222 variants. Fairly inexpensive--$35/pair on eBay. They do require adapters (which work for all the tubes mentioned) that are apparently available only from Chinese vendors with very slow shipping (at least 3 weeks); I paid $17.50 for a pair.