Looks like somebody just went along the tops with a hedge trimmer to make them look tidy. Cutting the tops of shrubs again and again (such as to make a hedge all the same height) makes the lower branches die back and the top branches thick and full, eventually stunted. If you want a proper hedge with leaves all the way down, you need to prune with a tapering shape towards the top so the lower branches still get some light. So unless you're willing to carefully and selectively thin the tops, there's no hope of the bottoms filling in any time soon.
Underplanting sounds like the best choice to me too. Just don't plant expensive perennials like hosta too close to the shrubs. Remember they have fairly large, established root systems probably a foot or two away from the trunks/bases. So when you go to plant the new plants, dig a nice big hole, cut out any roots you see in there, amend really well because those shrubs are eating up all the goodies in that soil, and take really good care of those new shade plants to help them get established before the shrubs discover the new source of food and water! Large, well established plants will compete ok with the shrubs, but new plants will struggle without help.