As for compatibility with my stock Galaxy S3 with Qualcomm Snapdragon S4 processor or my brother's stock Galaxy S4 with Qualcomm Snapdragon 600 processor, semi-ok. Out of the box it connects, but the audio is on fast-forward and you can't listen to that. I downloaded USB Audio Recorder Pro and with that it works and sounds AMAZING via either phone. I can't say if it's the D42, U-ARP, or the synergy with my V-Moda M100s, but it's a NICE package. Usability is a joke though, so I was still pretty unhappy with the compatibility and started thinking...
I thought there was some kind of access or authorization issue, so I enabled USB Debugging mode in Developer Settings and then reconnected the D42. Now it works great and can be connected to any app from GoneMAD to YouTube on both phones! The S3 is mine, so that's what I spend my time on, but the sound is the same, as far as I can hear, on both phones. This really helped, now it's a nice package and I get all the Album Art, Shuffle, Lock-screen Widgets, Playlists, and usability that I'd wanted in my portable audio solution. I'll be buying a new MicroSD-hc card so that I can start rolling this package for real and getting my money's worth
It'll be a while before I start to swap in and out the op-amps and buffers from the TopKit, I've got to get familiar with the combo before I start messing with it.