Bluetooth Headphones to work with my 2009 iMac?

I have a late 2009 iMac, 3.06Ghz Intel Core 2 Duo, 12GB memory, and running El Capitan 10.11.3. I understand it has bluetooth 2.0, meant for only the wireless mouse and keyboard, both of which I have.

I need new comfortable, wireless, over ear headphones. I tried B&O 4.0/4.1 bluetooth wireless which paired with my iPhone 5S and the sound was perfect. I then paired them with my iMac, they paired ok but the sound was awful, very rough and impossible to listen to. I returned them.

My previous headphones were Sennheiser wireless TC130 (I think) - they had a base which was connected to a USB port on the iMac and charged the phones when not it use. They were good but not very versatile so when they died completely I thought it was the time to move on to bluetooth. However I am finding that more difficult than I thought. I am not technology savvy myself so tried to find out what would work.

A local Apple "specialist" told me to buy a new computer, or buy non-bluetooth headphones - not very helpful! PC World computer shop told me that any bluetooth headphones will work in my iMac as bluetooth is backwards (or forwards, I cannot remember which) compatible and there should be no problems. That was not my experience.

I happened to find you guys tonight - I didn't know that such a specialised forum (i.e. iMac, iPad, etc) exists. I do hope you can help me. The B&O headphones were gorgeous and comfortable but I realise there may be some others equally good out there. I am retired and certainly cannot afford a new Mac until I absolutely have to.

I look forward to any advice you can give me.

Anne (Scotland)