Taking Contango's & Davidad's suggestion a little further - 22/32/42, with a big ring the same size as your your current middle one, is a common MTB setup. That would extend your gear range down by 36%, compared to 17% for the cassette change you're considering. You'd still have a 91-inch top - the current top gear on your middle ring, which as you've found is plenty for many people. And you'd be free to change the cassette as well later on, if you decided you wanted to go lower still.