Hello Tom,
Well I played with IMD today, played in that I visually read the figures as well as looked at the trace as I varied input level. Nothing to note that would point the finger at tape doing something to the sound, values similar to THD. Today I used RMGI 900 tape. Though this tape has NOT been calibrated to the deck, THD at various levels was running 0.15%, (SMPTE standard RP120-1994) IMD 60hz / 7 khz 4:1 level ratio raised THD to 0.25~0.3%.
You talk of tape saturation possibly adding clarity, I stay away from saturation.
Sean