Rebi,
It does sound like a different powercord is worth trying (I won't specifically comment on the old computer cord experiment). I picked up my Cardas Golden Reference PC for $225 used. There are a good number of used power cords listed on Audiogon, and that's one of the safer things to buy used in my view. You may get more bang for your buck that way. I have heard good things about the Pangea cords but have never tried them myself.
And Charles, it's good to know that the EML XLS tubes are a drop in replacement. They sounded significantly better than any other tubes I tried in my Diavolo. They are not dirt cheap (around $700 a pair new I think), but I was lucky enough to find a used pair listed here and from a seller 20 minutes from my house here in Denver. I'll never look back.
Rebi, you should definitely consider trying a pair of the EMLs as a less drastic measure, particularly if you really like your current speakers. I don't know the technical details, but I think the whole point of the 300B XLS tubes is to provide the magic of a 300B with a little more power.