The only tubes I've used in a V4 are NOS 7027a, (RCA, GE, Sylvania). You get plenty of clean headroom with these tubes. The V4 puts around 540 volts to the plates, so that's why I like to stick to NOS 7027a, which have no problem with the voltage. I've used the JAN-Philips 6L6GWB tubes in a Tweed Bassman clone, and they sound great and do give an early breakup. However, that amp has 460 volts to the plates, and I would be afraid to stick those tubes in the V4.