You could use a Mu-so Qb via Airplay, Bluetooth or optical S/PDIF with your iMac, but I'm not sure it makes a lot of sense. Also it's not possible to use two Qb units to create a stereo pair.
A better option would be to use either a set of passive speakers and an amp/DAC or a set of active speakers with onboard DAC. For example:
- a Naim DAC-V1 / NAP 100 and a set of passive speakers, connecting the iMac to the DAC-V1 via the asynchronous USB input.
- The NS10 inspired Yamaha NX-N500 active speakers. These have a ton of interfaces including asynchronous USB, Airplay and Bluetooth. Computer Audiophile has a recent review.
- In my case I'm using a NAD D 3020 integrated amp/DAC and a set of passive Q-Acoustics 2010i speakers with a 27" 5K iMac. The NAD has asynchronous USB for the iMac. It also has Bluetooth.
I use a standing desk so it's not really practical to put an amp/DAC under the desk. Despite having a 200/90cm desk something like the DAC-V1 / NAP 100 would take up too much space. I'm very happy with the setup, but if I were buying today I'd give serious consideration to something like the Yamaha because it means I can get rid of a box on my desk.
A couple of final things. A decent set of speaker stands can help in a desktop setup. IsoAcoustics have some good options. Bluetooth. I didn't think I'd use the Bluetooth connection my NAD D 3020 but as it turns out it's great for hooking up an iPad when I want to follow the test match or cycling while working :-)