I too have a 6505+ 1x12 and I think I know what's up. Its very tempting to turn that res knob all the way up, it produces what SOUNDS like a gratifying tone, especially by itself. A few things are at work here. This amp is strange in that if you have something in the fx loop, it changes the gain and sound a little bit, giving things what I can only describe as more of a high end sizzle type of characteristic. Couple that with the res knob being turned all the way up, and I think you have your culprit. I gig with mine regularly, my other guitar player plays a shit ass line 6 spider iv stack, and when I turn the res knob all the way up it makes my amp more like his; trebly, too much presence, sounds cool alone and shit in a mix. I would say back off the res knob until you hear that saturation fading out, and then play with nothing in the fx loop. Are you playing on the lead or crunch channel? If you have a ts I would recommend using that on the green crunch channel, if set properly you should attain the classic 5150/6505 sounds you hear around here. Hope this is of some use to you, good luck man!