The amp uses four 6C33C-B output tubes. They cost $160 a set at The Tube Store . There is only one company that makes them so there are no other options to get different sounds. lucky for you they do sound warm. There are several manufacturers of the eight 6SN7 signal tubes this amp uses. They do not have to be matched pairs. I have not had to replace mine in my VK-55 yet so can,t help you there. I too would be interested in ideas on a warm 6SN7 if anyone else is following this thread.