Sorry, completely forgot I'd posted in here until today. I just did a few tests using the E6 with the Sennheiser HD555 and the VSonic GR07. I ran everything out of the headphone out of a FiiO E10 since I don't have a tv with a headphone jack and the line out on the E10 sounded really static-y with the E6 for some reason. Splitter was one from Micca Store that came free with the E6.
The best setup I found was with the splitter plugged directly into the headphone out, the HD555 plugged into the splitter, and the GR07 plugged into the E6 (plugged into the other half of the splitter). With this setup, you can set the source volume to fit the full-size can, then use the E6 to bring that down to a proper level for the IEMs. With the splitter plugged into the E6 (E6 powering both), the volume was always either too loud for the IEMs or too quiet for the full-size phones. With the opposite of my final setup, the E6 couldn't get loud enough to bring the full-size phones to the same level as the IEMs.
Final setup looks like this:
______________E6_______________IEM
|
SOURCE------------------|______________________________FULL-SIZE PHONES
Hope this helps!