I have owned my 2555x for a couple years now and am curious about trying some different tubes. I have heard that the tubes Marshall uses are not very good. I am guessing that messing with the pre-amp tubes will probably make a bigger difference since I rarely run it past 6 or 7 on the master. ANyone have a tube combo they really like? I am looking to get a decent amount of bottom end and maybe something that does wel with high gain.