The only real difference I have seen are:
- A8A has 2 extra channel amps, if you want 7.2.4, or bi-amp with either 7.2.2 or 5.2.4; however the A6A does full 11 channel processing and has pre-outs so you can add an amplifier for those channels now or in the future (which will probably be less than £1000.
- The A6A uses the same DACs as the 3080 did (ES9026PRO for LCR and ES9007S for surrounds), but the A8A upgrades the secondary DAC to ES9026PRO
For the first point you probably know up front if you NEED those two channels now, or in the near future, or not. You either have or are building out to 7.2.4 or you aren't; you either have speakers you can bi-amp and want to, do it or you don't (and whether its worthwhile or not is a whole other discussion). You either have or want to have zone 2/3 setup and want to drive it from same receiver, or you don't...
For the DACs... it's trickier. How much does this matter? It's really hard to say. The overall engineering of the AVR probably matters more than the DACs themselves, and the quality of your speakers matters too - a great DAC probably won't output better sound through average speakers. Plus, the content you put through them - there's only so much a DAC can do, if the source was mediocre to average encoding/over-compressed/etc (e.g, anything sent over standard bluetooth audio, most streaming services, etc).
Other than that, I believe they have exact feature parity - but the A8A weighs 6 pounds more, and is nearly 1.5" longer (depth).
Edit - I think there was one other difference I saw, now that I remember, that the power supplies is split for the main unit and the amplifiers in the A8A. That is a nice improvement. Worth the price difference alone if you don't need the extra channels or care about the DAC as much? I can't say.