I'm not sure one word will do in all contexts.
The best "expressor" at work could be that aspect of the workplace that
best indicates the prevalence of well-being: why not "the best sign of
well-being at work"? If you mean the aspect that produces the most
well-being, perhaps "greatest producer".
I take it that the soul as "expressor" would be the agent that offers or
communicates love to another person: in that case, I might be tempted to
use "the soul as giver of love".
If you want to use an agent-noun from "express", I think it should be
"expresser", the English formation from the English verb meaning "to
make plain". The other form, "expressor" is the Latin agentive noun
from "exprimere", to press out; I would use that literally for someone
or something that extracts juice or oil from some substance (olives,
sugar-cane, fruit).