"I have a pair of Zu Souls MK11, I like them, but wish, I have more bass slam, and a slight reduction in the treble, only just!!"
This thought occurs to me:
You can think of the tonal balance of a speaker like a see-saw: If the top end is too much, you can add more bottom end (via subs) and get a really nice balance. But if you add subs to a speaker that is already nicely balanced, that can make it sound unbalanced, like it now needs more top end to regain balance.
So your Zu’s might be inviting you to add subs, which may accomplish two things at once: Give you more bass slam, AND balance the see-saw, so that you no longer need a slight reduction in the treble.
(I used to be a Gallo dealer and have enormous respect for Anthony Gallo's innovative designs, but the Zu's have a really enjoyable liveliness that I think you would miss, hence my suggestion for a way to keep what the Zu's do well and improve the areas where there is room for improvement.)
Duke