REW V5.17 beta 11 has added the capability to support multi-channel output using Java drivers. The Windows JRE doesn't offer multi-channel even when using an audio device capable of it, but OS X does (tested on 10.11.6) and Linux might also (not tested). There is more info in the updated help. Note that I have labelled the channels 1-8 in order as Left, Right, C, LFE, SL, SR, SBL and SBR, but they may not always be driven in that order - the soundcard I tested on OS X has the side channels on 7 and 8 and the back channels on 5 and 6. I have kept a 'Both' option which drives Left+Right. More info on the changes in this release on the Beta Versions page.