I seriously doubt anyone would ever try to drive regular speakers using the headphone output. It's not a thing.
Regarding Arcam, their gear runs the gamut. They make some amazing sounding high end stuff (dCS Ring DAC), and middling performing gear at the lower end of the price spectrum. Which model Arcam and which speakers were you using? The only reason I can think your experience of using the Mojo in a standard 2-ch setup was less than satisfactory was because of the downstream equipment. Also, did you lower the output for the Mojo below 3V as mentioned previously? The reason I asked why Chord chose to use 3V for the line output is because often times you will find hardware that does not play nice with >2V and will distort if the input signal exceeds 2V.
'I seriously doubt anyone would ever try to drive regular speakers using the headphone output. It's not a thing.' - If you read this Mojo thread from start to finish, you will discover many Mojo owners pushing the boundaries of what was initially thought possible.
For my amplifier experiment, I fed the Mojo output into an Arcam Solo neo, and the output of the Arcam was fed to a pair of Focal Aria 936 floorstanders.
The sound lost everything that makes the Mojo sound special, the instrument spacing, the music transients, and the clear sound.
I am pretty sure that I tried adjusting the Mojo volume, but no improvement was possible.
I never tried direct driving the Arias with the Mojo, because I assumed that there would not be enough power.
However I have used a Hugo 2 to direct drive the Arias, and I am delighted with the sound, and have no interest to buy more expensive speakers, although it would be tempting to hear some electrostatic speakers.
I never expected that the Hugo 2 could drive the Arias which are rated at 300W max.
So it was a big surprise that the Hugo 2 with the volume set at approx 50% (so approx 1W) could drive those speakers.
The sound was perfectly loud enough for me to listen to jazz music, but I am aware that for louder volumes the needed power would increase exponentially.
The conclusion was that there is no problem with sound quality from using the Arias, and that for many purposes (especially near-field listening) you don't need many Watts of power.
I have also used the Hugo 2 to direct drive Wharfedale Diamond 1 and Monitor Audio Silver speakers in near-field mode, using less power than for the Arias.
With this low level of power, we are entering the range of power levels possible from the Mojo.
I have never tested this scenario, but I anticipate that it would be possible to direct drive efficient speakers in near-field mode with the Mojo.
I think the Mojo thread contains posts from owners who direct drove computer speakers.
Returning to the original topic, I believe that the Arcam was the weak point in the Mojo>Arcam>Aria chain.
I decided that it would be pointless trying again, unless I used a much better amplifier.
Here is an image of the Hugo 2 driving the Wharfedales (but the Monitor Audio are much better). I anticipate that the Mojo could do a similar job.