The first thing I would advise is to do nothing at all.
As the gear is brand new, familiarize yourself with the sound before buying additional gear.
The moment you are thoroughly acquainted with the sound, that is the moment you can really judge what an additional component can do for you.
Subwoofer.
A lot of people like bass, in fact it can’t be load enough.
In case of classical a bass heavy system is a disaster.
It simple destroys the balance.
If possible, hear a symphony orchestra live.
Listen carefully to the double bass and the kettles drums.
They sound like ‘Zoom’.
On a lot of gear they sound like ‘Boom’
With a sub it often becomes ‘BOOM’
DAC
iMac analog out is not the best there is.
But a lot of cheap DACs do not really improve much on the on-board audio.