Two things you might want to try:
1) Find where you can set the speaker size for the speakers on your system (Crossover settings). By default, your "Surround Back" speakers will be set to "None Main". You'll want to change it to "Small Main" or "Small".
2) Find the surround settings for Dolby Digital, DTS and Logic 7. In these menu's, you should be able to specify 5.1, 6.1 or 7.1
I'm not sure where these menu's are on the 635, but that's what I recall from my 525.
Hope this helps!