If the output tubes can't be brought up to their proper bias setting; they're shot and you've lost your dynamics. Replace them! If you were happy with the Siemens when new(excellent tubes), purchase another set(or at least a set of SEDs from the St Petersburg plant). ( http://www.upscaleaudio.com/bEL-346CA7-Typesb_c_86.html )
Groove Tubes EL34LS vs SED
i have a amp air tight ATM-3 to be precise that currently uses Siemens EL34. I want a bit more dynamics especially at the bass. Has anyone tried these? The Siemens seem at the end of their life, although they test fine in a tube tester, the bias cant be adjusted sufficiently high. Since I got this amp recently (used) I dont know if the lack of bottom end is due to old tubes, or just the amp itself.Advice appreciated THX