I've never used a mixamp myself, so can't comment. User comments seem to be the usual sound quality mixed bag with those who have already shelled out for one vs those who are happy with their current setup. Probably one for the audiophiles.
how much does a controller cost?
like £25 to a retailer. so they are being made for less than £10 each.
that means the parts are the cheapest they can get. there is no way it will have a powerful AMP inside the controller to power them properly. you then have to factor in what the quality of the bluetooth controller will be like. the old ps4's were hit and miss i know my original ps wasn't made right and the BT controller was faulty. xbox will have similar issues. then you have to think about how good will the DAC be inside the controller too?
all of them will be like £1 each max. so cheap crap.
it may sound fine to someone who doesn't know what real audio quality is. but then why are you spending £100+ on a set of headphones but then using £1 parts as the source? why not just plug in a cheap £20 headset? save yourself £100.
i don't even rate the mixamp too highly the AMP is very weak. more than powerful enough for sennheisers but it won't power AKG's which are power hungry. where do you stop? you have to somewhere because of law of diminishing returns. however the mixamp is very good for what it is and i don't think £50-£100 is too much to ask to spend on one if you want decent sound quality.
you can buy them second hand they are pretty well made.