M2i dont have as much bass as M22ti or M3ti, and stronger midrange than M3ti, if you're going to put them on the shelf then look no further than M2i, M22ti has much stronger mid than both M2i and M3ti but you'll more than likely experience the same boom or excessive bass as the M3ti, so I say M2i is a much better option for you.
Or, use the Tangents as your bookshelf, and get the M22ti as replacement.