I use two REL S/510 for two channel. My mains are rated fairly flat down to 28hz, but the subs still produce a notably fuller sound. They're punchy, not boomy. A few days ago, I moved my gear to a new rack. When I turned it on to test everything, I could tell that I had forgotten to turn the subs on. The gain is set quite low as is the crossover frequency. They blend well with my mains.
I have a larger, ported SVS sub for home theatre. It wouldn't work well for two channel. I'm sure SVS has better subs for music. Mine shakes the house when watching movies.