First of all the DT 770 PRO and DT 770 (Edition 2005 - Consumer Version) are "related".
The differences are the headbow (more tension on the DT 770 PRO), the housing (causing both headphones to sound slightly different) and the packaging. The different impedances:
- DT 770 PRO in 80 or 250 Ohms (32 and 600 Ohms are special versions and not available for retail)
- DT 770 (Edition 2005) in 32, 80, 250 or 600 Ohms, not all available for normal retail, but all available through our MANUFAKTUR.
The differences in sound between the different impedances is theoretically simple:
Headphone systems or drivers with lower impedances use slightly heavier wire for the coil and are therefor not moving as rapid and exact as the higher impedance systems. Conclusion: lower impedances mean slightly heavier membrane + coil --> slightly more mass --> slightly slower to move causing headphones with higher impedances to sound slightly more detailed.
Concerning the MMX 300, this is actually a seperate product but if you want to relate it to a DT 770, than a DT 770 (Edition 2005).
I hope this removed your question marks?
Thanks and best regards from Germany,
Peter Grooff
Product Manager Headzone® / Headphones / Headsets / OEM ProAudio
beyerdynamic GmbH & Co. KG