Mesa Tubes Bias help

Okay so I bought a Mesa rectoverb head series 1 from my local Guitar Center recently. The guitar amp has been modded slightly it has an el34 6l6 aftermarket switch. It also has a jack labeled vintage modern on it which could be a foot switch according to Mesa. The guitar amp did not have Mesa tubes in it. However it shows no sign of a bias mod except for the switch. They say made in Russia el34 0306 and I think they are Svetlana electron tubes. When I took it apart I could not find a bias pot. Are those tubes within the Mesa fixed bias range? Or are they possibly just thrown in there to work? Currently everything works fine but I'm more of a fan of 6L6 tubes I would like to put a matched pair of Mesa tubes in there. Any help would be appreciated.

Anybody?

Without knowing what mod was done, there is no way anyone can really tell you much. I would take it to a decent tech and have them look it over.