Placing it nearfield like that can give you more tactile feel since it will be firing right into the back of the sofa but it still doesn't change the fact that it will still be trying to fill all open space.
Did you do a sub crawl when finding the best spot for the sub. Sub crawl in case you don't know is, place the sub in your main listening position and then crawl around the room and find the spot the sub is the loudest.
As for the overall sound of the sub, you can only expect so much for so little. If you want articulate, clean, flat bass you need to go up in budget and the best deals are from the internet direct sellers like SVS, Rythmik, HSU, PSA, Outlaw........ or build a DIY sub.
Klipsch RF-62II, RC-500, RS-400, SVS PC12+,
Def Tech SC8000
Harman Kardon AVR 1600
PS3, Apple TV, Sharp 70" Qattron