Can't really recommend anything without knowing what type sound you really want. What tubes are in the Croft now? What's wrong with them? Anyhow here is a link to Brett Jessee's web site where he compares sonic signatures. But remember the topology of the amp can make a huge difference in the sound signature.
http://www.audiotubes.com/12ax7.htm
http://www.audiotubes.com/12ax7.htm