JBL headphones have miserable sound quality with bluetooth on laptop, deactivation of mic (the supposed fix) is not possible

I have a set of JBL Headphones too, though not exactly your model. I was facing similar issues as yours and coincidentally, just like you, none of the solutions from the thread you linked worked for me either. I also thought it was a hardware problem but that turned out not to be the case, at least for me.

To clarify, for me the issue was that when I connected the headphones to my phone the sound quality was perfect. On the other hand, when I connected them to my laptop the sound quality was noticeably lower. There were some audible high frequencies in some songs, in other songs the sound was somewhat muffled and tinny - almost as if there was some noise reduction being applied to the audio. It is a little hard to explain because the defects were quite subtle. But, in short, the sound was definitely not as clear as it was when connected to a different device.

What Worked For Me

First of all, I'm using Windows 10 and I believe my Bluetooth hardware component is by Realtek.

Back to the fix: it was actually a driver issue but hear me out before you say "Aaargh! Tried that already!". I'm sure you have already tried to uninstall the driver several times by now. I did too.

I realised one thing: whenever I reinstalled the driver, my laptop would still get the faulty one as the latest or best driver. So, after experimentation, I found the following steps fixed the problem:

1. Unpair/Remove the Headphones

   I'll post the steps as reproduced on Windows 10, but I think some of them are the same on Windows 8.1.

   While the Bluetooth is still on:

   Windows Key → Settings → Devices → Bluetooth & other devices → Right-click on the device then click "Remove Device"
   (Will be greyed out if Bluetooth is turned off)

2. Turn off the Bluetooth on the Laptop

   This is the first crucial step I missed when reinstalling the drivers the first few times.
   To turn the Bluetooth off, click the notification icon on your taskbar and then click the Bluetooth icon, making sure it is not highlighted. Alternatively, you can use the function keys or hardware switch if your laptop gives you those options.

3. Go to the Device Manager

   One of the quickest ways to access it is by using "Windows + R" then type devmgmt.msc and press enter.
   Next, find and unfold the option that says Bluetooth. If your Bluetooth is off, there should only be one option under "Bluetooth". On my laptop, this option is Generic Bluetooth Adapter.

4. Install the Local Driver Called "Generic Bluetooth Adapter"

   This is the second and final crucial step.
   From step 3...

   Right-click on Generic Bluetooth Adapter → Update driver → Browse my computer for driver software → Let me pick from a list of available drivers on my computer → Make sure Show compatible hardware is checked.

Now, there should be a list of drivers that are locally available (there were three in my case) and one of them should be Generic Bluetooth Adapter. Select it and install it by clicking next. After the installation, turn the Bluetooth back on, try to reconnect your device and see if the problem persists. If it is still not fixed, maybe at this stage try to disable "handsfree telephony". If this still does not fix the problem, repeat the steps but this time select the next driver on the list when you get to step 4 (if there was more than one option).

Hope that helps.