I don't have an Acanthus, though I have been wanting one since I saw it planted in a bed at the Science Center in Seattle a few years ago, lol. Fantastic looking plant.

If it were me, I would probably pay most attention to the foliage of the companion plants. Personally, I would pair the Acanthus with a plant that has strappy/grassy foliage as an opposite texture and/or a plant that has delicate or ferny foliage to act as a softening contrast to the bold, jagged foliage of the Acanthus.

A small Miscanthus ('Little Zebra' would add some nice looking gold variegation too!) for beside it and then perhaps a perennial geranium (like G. sanguineum 'Vision Violet', which has nice looking delicate foliage) in front.

But that is just what I might do ;-)
CMK

I like that suggestion a lot. One of the plants I considered was Rozanne geranium, but your suggestion of a variegated small grass would fit the bill even better. Thanks!