Agree that they're not a substitute. However, they do need to be considered when looking at the appropriate amounts of what kinds of treatments in a room. People, furniture, carpet, wall construction, doors, windows, etc. - all have some absorbtive qualities at various frequency ranges.
The changes you described will not make a lot of difference though. You'll actually have LESS absorbtion in the room now than with the old furniture overall. Though, the cloth couch will provide a little more upper end control. The thing is, when you're sitting in the couch, you're blocking most of that anyway.
Bryan