Do you know if yours is the v.1 or v.2? It won't say on the unit, you have to call VTL w/the SN to find out. And does yours have the phono stage?
Sorry, I don't know about diff. tubes, but you can call VTL & usually talk to Luke's wife I think, very smart & helpful.
After 5 years, I replaced mine with a Cary SLP-98 (at twice the cost). But I sure enjoyed the 2.5 TL while I had it.....