Well, you know I had to try it; I put the Mystere in as the main for the SC-IV's and played it in Stereo mode!
It confirmed some of my original thoughts. With the Siemens RFT EL34's the Mystere doesn't drive as hard as my Rogue 90 with the Psvane KT-88's. But I think it has more to do with the higher input impedance of the Mystere compared to the Rogue (100kOhms vs ~70kOhms), rather then the nature of the EL34's.
What is interesting is the Mystere has more voltage gain in the output (~32db vs ~28db), but as John Atkinson determined in his measurements of the IA21, the output impedance of the Mystere is "interestingly" high, so that may have something to do with the drive from the amp.
Also, I now get Disk Olsher's TAS Review of the bass presentation. It wasn't as solid and locked in as the Rogue. I could also detect the roll off of the bass and of course the highs seemed a bit smother because of the EL34's. But the Rogue definitely is more dynamic (faster response) and that's another element Olsher pointed out with the Pentode output coupling design.
Bottom-line, the Mystere in my system is more suited to what I envisioned having it drive the HRCC center channel. It does voices so incredibly well with the EL34's. I definitely will keep the Rogue running the Fronts (mains), but I may experiment with rolling the KT-88's out for some KT-77's, that might be interesting!