Better than APTx is not really necessary I think. For something like the M50xBT it would be completely pointless.
Very likely the hardware simply doesn't support any further significant upgrades. If was to criticise their support of the X7, it was that the newer versions of the firmware ran slower than the original versions. I wouldn't be surprised if the lock-down on installable apps on the M9 was the result of customers complaining about the X7 not being usable like a smartphone.
Speaking of which, it's not a popular app, but I'd put my hand up to vote for
Radio Paradise being whitelisted. It can, at least on a regular phone, cache many hours of their playlist in FLAC format. I find it a good way to discover new music.