Call the manufacturer and ask how many volts are going to the plates of the 6L6. If it is not more then 320v, try some NOS RCA,Tungsol or Ken-Rad 6L6G. These have a very different sound then the EL34's and once you hear the difference you might never go back to the EL34's. They are very musical and give wonderful body to the music. Please check with Esoteric first. Leave the 6L6GB and 6L6GC alone. Try the 6L6G or 6L6GA. They are a bigger tube being the shoulder variety so make sure they will fit in your amp before buying a pair or quad.
Good luck,
Bill
Good luck,
Bill