Hey NARSH if you have not got MIDIWizard by Graham Pearson (gsp-2101@yahoogroups.com) for the 2101 I highly recommend it and the second PPC as well. I went through the the process of trying different tubes in the 2101 (Doug at www.dougstubes.com made up a sampler set for me, highly recommended) and found that it made a subtle but very noticeable differences, I wish I could remember where I stored my notes! I seem to remember liking the Mullard, but can't remember if that was what I settled on..