The VH4 was originally designed for EL34 and works best with those. I talked to Peter Stapfer about putting KT88 in and he told me he tried once but didn't like it at all. Way to cold and stiff sounding. But he said KT77 were great. I wouldn't recommend using 6L6 in Diezel Amps. The dirties suffer a lot though the cleans are a little better.