Sorry for delay, so I was trying two things at once..
• Meta_query without post_object field in a group
• Meta_query on a post_object field that is not multi-select
The first is obvious as you pointed out but is dependant on the second
Am I correct in thinking that for the second I would need to omit the double quotes?
'value' => $instrument,
..Instead of..
'value' => '"'.$instrument.'"',
What about compare? Should I use LIKE or IN for single post_object?