I'm not sure what you're looking for here - I think you have answered your own question...
- You know that the impedance going into the Axe is 1Mohm (albeit adjustable)
- You know that the impedance going into the RME is 470K ohms (less than half)
- You state that higher (1M) impedance gets you a fuller, higher amplitude signal
- You state that lower (470K) impedance gets you a tinny sound with less amplitude
Unfortunately you cannot set the Axe to 470K to see if it sounds the same as a the RME.
This almost seems like the old story/joke:
Patient - "Dr. - it hurts when I twist my fingers"
Dr. - "Well then, don't twist your fingers"
Guitar Player - "My signal sounds tinny when recording/DI'ing through the RME"
Audio engineer - "Then don't DI through the RME (use the Axe instead)"
Solution 1: Simplest solution appears to be to use the Axe to record your DI signal digitally (i.e. via USB).
- ADVANTAGES: No extra hardware required; you can record without taking up extra channels on your RME
- LIMITATIONS: You must record at 48Khz
Solution 2: Another possible solution - get a real DI box that's made for guitar (I use a Radial JDV) and let it manage the impedance the guitar sees. The Radial's also have an adjustment called 'drag' that lets you adjust the load on your guitar's pickups.
- ADVANTAGES: Record at any sampling rate/bit depth you want
- LIMITATIONS: Costs more than the first solution; Takes additional I/O channel(s)
Good luck!
:eagerness: