Bear with me here as this is my first tube amp. I am looking to replace the tubes on my Bassman clone. Currently it has an EH12AT7 in the first position and two Sovtek 12AX7s in positions 2 and 3. The power tubes are two Fender Groove Tube Code Reds 6L6s. Any brand recommendations to replace the tubes? I am hoping to have the amp break up a little earlier. Also can I replace the tubes myself or do I have to adjust the bias (still a little unfamiliar with how this works). Just don't want to mess anything up. Thanks for the help!