Hi Naveen,
I have auditioned Onkyo 3100... and ultimately purchased Onkuo 5105. I have faced the same issue including the fact that the bass sounded good in demo-room. Here is what I have to say...
1. The cross-over frequency selection has BIG impact on Bass sound. For your 3200 HTIB, keep it at 120Hz and set your fronts as "Small" speakers.
2. If you have "Audissey" function, use it right away but follow the manual instructions STRICTLY. You will notice overall bass enhancement that is better than what the Bass control offers.
3. If your AVR has EQ setting available for Subwoofer, tweak that if you find the Audissey function has not given desired result.
4. Try shifting around the subwoofer AFTER completing till point-3 above and then observe the difference. (This whole affair takes patience man!! I took two weeks to get the best possible)
4. The demo room I visited was small about 9 feet x 10 Feet, my listening distance was 3 feet, room was full carpeted having heavy curtains. So the AVR filled the demo room with sound effectively (see how powerful even a 25watt car stereo sounds in a car... because the car interiors are small and your listening distance is small... plus the interior is heavily padded with foam-seats, floor carpet and roof-fabric) When I returned to my living room 16 feet by 18 feet and installed the AVR at 60Hz subwoofer cross over, I was shocked at the weak overall sound. Maybe your case is similar to mine.
5. Download some woofer testing files from here and observe results >>
RapidShare: 1-CLICK Web hosting - Easy Filehosting
You may find that subwoofer is doing fine in some low freqs while weak in others. Thats the case with low end HTIB subwoofer. To be fair to Onkyo, their subwoofer (and overall 5.1 output) is better than Sony HTIB in the same price range.
6. Human perception: Now this is most interesting. You may be surprised to know that our ears are prone to hear differently on different days. So one fine morning (usually its morning..) you will suddenly find something lacking in sound, and wonder what happened since last night when everything was perfect! This is just for info, ... it doesnt happen on everyday basis.
7. I see bookshelf speakers behind your front speakers (in your image provided). If they are 8 Ohms each, try using them as fronts (change the Fronts setting in AVR as LARGE now). That should help in bass also.
I hope you haven't dozed off by now and the above helps.
Regds,