So, I replaced the tubes in my IRT Studio and now I cannot get anywhere near the recommended bias settings. I tried two sets of EL84 (JJ and EHX) and for both, the maximal value that the bias pot will allow me is about 22mA with a plate voltage of 240, so a good deal below the "suggested" settings. Judging from the position of the bias pot before the tube change and also from the sound of the amp, these settings are still too "hot" for the amp though. The sound becomes rather woofy with lots of low end and clears up when the bias is set at around 10mA. I was wondering whether there is an "explanation" for this or whether or not I'm missing something?