OK, update.

If I bypass the HDFIX22 and go direct from the Amazon FireHD to the HD621 I get Dolby Digital and my 861 switches to Cinema mode. With the HDFIX22 inline, I can only get PLIIx Movie. Looking at the manual it looks like the HDFIX22 supports DTS MA/TrueHD over HMDI but only 2-channel PCM, which is probably the cause of the problem smirk

Will let you know how I get on with the HDFury!