MB 12" Retina - Poor bluetooth audio input quality

I upgraded from MBA Mid-2011 to MB 12" Early-2015, full specs. Both on fully upgraded Yosemite (10.10.5). Voice recognition using as bluetooth input device Voyager, Jawbone ERA and Jabra Revo worked well on the MBA. It no longer works on the MB 12 as the quality of the voice is insufficient (crackling and very metallic). Same in Qucktime recordings (when they succeed, as recording with bluetooth source crashes the app more often than not). Already tried:

- resetting SMC

- resetting PRAM

- resetting Bluetooth module

- fixing permissions

- resetting the headsets (all the firmwares are up to date)

- working on the bitpool rate through the bluetooth explorer

all to no avail.

Apple Diagnostics shows no issue.

Input quality using a (albeit very old) external bluetooth dongle is good enough but it is not an acceptable workaround when you only have one port. this seem to be a software issue very specific to the MB 12 Retina. Any clue on possible solutions?

Thanks,