Originally Posted by bobnegi
My speakers(Wharfedale Diamond 9.6) go down to 20Hz.
Ummmm... first of all, your speakers do *not* go to 20 Hz. Their response is -6 dB @28 Hz. The tune point is 30 Hz.http://www.wharfedale.co.uk/model.php?model_id=16
This means they are down 12+ dB at 20 Hz and are clearly not "full-range" speakers. (Don't get me wrong, these are great spec's for any speaker, but they don't make your speakers into great "subwoofers".) Add to this the fact that the best in-room position for imaging purposes for direct radiators is well away from the side and back walls, (which is the *worst* possible position for bass reinforcement), and your speakers appear to be even less attractive for really deep bass response.
If you set your speakers to "Large", you will be sending them a "full range" signal. Since they are not "full-range" speakers and they are probably not placed for optimal deep bass response, anything below their LF extension, (the tune point of 30 Hz) will be attenuated significantly. However, if you set them to "Small" the lowest bass will be re-routed to the sub and not "lost" or attenuated. Assuming that your sub is the best speaker in your system to reproduce the lowest bass, your best bet is to set your speakers to "Small" and allow the 90 Hz crossover to re-route the lowest bass to the sub.
Then set the sub's crossover to it's highest setting, (or turn it off). You do not want redundant crossover filters in the system, as they will cause phase anomalies and frequency response problems. Let your receiver's Bass Management do it's job.
In the future, you may benefit from a more capable Bass Management system in an upgraded receiver. More flexibility in the crossover, speaker size, distance and level settings may be helpful also. For now, optimize what you have to work with.