For the audiophiles among you, I confirm that the Tab S 8.4 is USB audio capable. My HRT microstreamer USB DAC works perfectly.
There are many Y cables on eBay that allow charging and otg use at the same time. I find it's a bit hit and miss, as some don't work while others doThat is what I am looking after to.
If I only could get my eyes on a service manual I will find a way to charge while OTG is enabled.