Make the mono check answer a question
A mix whose lead, bass and main rhythm parts survive a mono speaker, without giving up the stereo picture.
The problem
A part sounds wide and full in stereo and drops away when the channels are summed. Phones, smart speakers and some club systems play in mono, so the question is which parts need to survive it.
What to do
Pick the parts that carry the song: usually the lead, the bass, the kick and snare, and one rhythm part. Play the busiest section in stereo, then in mono, at the same level. Listen only for those parts.
Expect the picture to change. Reverb and width get smaller in mono, and that's fine. Worry when an important part drops in level or changes tone.
When one does, look at what's making it wide. A stereo chorus is a common cause, because a chorus can't make a recorded double. On Taurus Chorus, set the MONO / STEREO switch to Mono, which sums the three voices, and listen to the part. If it thins out, bring MIX down or narrow WIDTH until the mono version holds, then switch back to Stereo.
Layered parts cause it too. A guitar doubled with a short delay, instead of a second take, can turn hollow when summed. Keep a real second take if you have one, or bring the delayed copy down, and listen again.
Change one thing at a time and check again in both modes.
When to stop
Not every part needs a solid centre. A pad, a reverb return or a percussion loop can be mostly decoration. If you're narrowing everything to protect mono, you're mixing for the wrong speaker. Fix only the parts that carry the song. Check the low end too, because a wide bass is the first thing to suffer.
The test
Play the chorus on one small speaker. You should hear the words, the bass line and the beat clearly. Then switch back to stereo and check the fix didn't make the mix narrower than you wanted.
Rory