Tube suggestions for Audio Research VT 100

What tubes would you recoomend to replace my original tubes for my VT 100. Also do AR still do upgrades from VT 100 to VT 100 Mark III Is it worth to upgrade? I have the LS 15 as the pre amp

The upgrade from the MKI to MKII is a major step up in perfromance. I own the MKII and prefer the tonal balance of the MKII to the MKIII which uses an hybrid input section (FET/6H30). Recommended output power tubes would be the Sevetlana 6550C. You should purchase a matched Octet. The input tubes (6922) must be very closely matched. I recommend purchasing the input tubes (6H23) from Upscale Audio. However, if you change the input tubes you need to propely adjust the bias which is not a trivial undertaking. I have done it my self and it is quite time consuming.