I'm thinking about changing the tubes in the Marshall JCM 900 SLX-100 Model 2100 I recently acquired and wanted to ask the experienced Marshall players for recommendations and suggestions. It's pretty dusty in there and I was going to open the head up and clean it up anyway so I thought while I was in there I might as well do a tube swap. Nothing's wrong at all with it, she sounds great imo, but I'm not sure when the last time she had a good going over was. I have several EHX 12AX7's virtually new here that I have stored away and a matched quad of JJ EL-34's I took out of my JSX head when I installed TAD 6L6's instead. Any pointers would be greatly appreciated. I've had tube amps many years but never a Marshall until recently. Not sure if I would need to re-bias and if so, how. In my town the techs are pretty unreliable. Thanks!