Hello, I reviewed the A83 here – http://www.hifiheadphones.co.uk/reviews/fidue-a83-review-hybrid-unbridled/

It’s hard to compare the two models as the A91 I reviewed is with another reviewer so I cannot try it right now. However the A91 is clearly better – there’s a solid yet effortless quality to the A91 where the A83 works a bit harder. Other flagship IEMs tend to emphasise the treble in the sound to accentuate the detail but the A91 is smooth yet detailed and very competent-sounding too.