What do I need to look out for to know if something has HD audio? Sorry I'm a real amateur.
Thanks
Like someone else said, you need to find out if these mini boxes support DTS-HD-MA and Dolby TrueHD passthrough. If they do, then all you need is an AV receiver that supports it (does yours?) and then you can enjoy the audio quality improvements over regular DVD quality audio such as DTS and Dolby Digital.
Passthrough means, the box just passes the encoded sound straight through to the Audio-Video-Receiver (AVR or Amplifier as it's sometimes called). The Minix Neo X8 is one of the top android boxes for sale right now AFAIK, and from looking at this site near the bottom right, it doesn't support any HD audio passthrough from the looks of things, so it's no good for your needs.
http://www.futeko.com/products/MP052.php
You also need to factor in future proofing. A new codec has been released that has the same level of quality, but only half the file size of the current most popular codec H.264 It's called HEVC H.265. You'll be wanting a device that can play this new codec in the future. XBMC has only just begun supporting it, and AFAIK there isn't any ready-to-go media players on the market that support it YET except for pc's.
I was in your predicament a few years ago. I wanted HD audio and the ability to futureproof to a degree without having to buy a complete new system. The answer (for me) was my old gaming rig, which i took out and put into a proper HTPC case which looks like an expensive top end amplifier. It's not the most efficient solution energy wise, but i only use it for home cinema move use so it suits me just fine, and is the most future proof option available. If you do go the HTPC route, i would get the most energy efficient components you can. It sounds like an HTPC is the best option to you, unless someone else can chime in with a recommendation on a ready-to-go media player that is capable of HD audio passthrough.