I like 700x28 Conti Grand Prix 4000S II's. On DT Swiss R460 rims they plump up to around 30mm. If you think you've got room for 32s, the Conti GP 4 Seasons come in that size and even on wide rims they don't go much over 33mm. If those aren't plush enough for you I don't see any way around the 650B conversion.