I play harp and have recently acquired one of these amps and I am trying to set it up for less feedback with tube swaps. My question is about groove tubes power tubes. I would think I wanted Blue for a dirtier sound but I am not sure if they mean distorts earlier by dirty. I am looking to have it distort at lower volumes. I would use Tung Sol 5881's but I have no where around me (boonies) to have it rebiased. Any advice would be appreciated.