While I do not recommend it to anyone else, I have used the 6F8G with great success in the Crack without Speedball. It runs cool and sounds great. For some reason, it does not function as well in the Crack with Speedball. I am not technical enough to address the issue, however, I keep on listening. I have never tried the 6F8G as both front and back tubes. I expect it would work quite well in the amp without Speedball. Good listening, JS