So, I had my 901 for a few days before the balanced amp card and RE-600 Songbirds arrived. I could not tell too much of a difference so I tested with the TRS adapter and no matter what switch position it sounded the same.
I then removed the TRS adapter and messed with the switch and not difference. I found that I have to push (the headphone TRRS connector is in all the way) very hard and it does go into balanced mode. Once it is in balanced the difference is major so it is not like it is difficult to tell when it engages.
My balanced switch has some play in it, at least a millimeter.
Does anyone else have this issue?
I have ordered a balanced HD-650 that should arrive today so I can test two devices.
Also I have used the Momentums and they do sound very good but of course I wish I could get a balanced cable for them.