Wow! what a dramatic beautiful quilt. My tastes lately have been leaning towards the warm color palette of reds, oranges. Your red on black is just stunning. Combined with a celtic knot motif you do have a real winner there.
I agree that the fabrics and colors you chose do speak quite enough with out extras. So I would make the quilting subdued on this as well. I completely agree with Tesspug that some SID and duplicate the embroidered knots in your black border would look stunning. I would be half tempted to create narrow bias strips and hand applique them. Just beautiful. Please post a picture of this when it is finished.