Just got off the telephone with Scott and I'll be checking the input questions he posed while at a job this evening. I believe the input volume of the Harmony-M will have an impact on the overall output voltage, but I'm not quite sure. I'll post an update tomorrow morning.

As for an inline amp, Radio Shack sells a 3-channel Headphone Volume Booster that does a pretty good job. I think I paid about $29 for one five or six years ago. They now have something SIMILAR

Gary cool
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!

K+E=W (Knowledge Plus Experience = Wisdom.)