I do not believe your current speakers from your Sony system would be compatible with the receivers you've listed unfortunately. YMMV.
Dollar for dollar, Onkyo presents a very strong value.- even their HTIB systems. Sony is a little overpriced IMHO and E. That particular Pio should perform well too. You would also need to spend more on speakers- ideally, more than the receiver since speakers are the most important part of an audio system.
If possible, I'd budget a little more $$ for separates. Boxing matches certainly do not require high audio fidelity. Grab a budget receiver and in a year, find some 5.1 speakers & sub that fit your budget at that time. Paradigm makes some nice affordable ones that Amazon carries. Heck, rent a surround system for the night just to tide you over. Maybe a friend would split the cost with you. I've done it before when we needed something for the kids to use when the adults were monopolizing the primary HT equipment at a new homeowner welcome to the neighborhood party .
I currently I have a Sony HTDDW790 and I need to upgrade it before the Maywether vs Mosely fight. When I bought this system, my living was about 150-200sqft and now my livingroom ( new home) is roughly 400sqft. I'm looking to spend in the neighborhood of 300-500$ I'm looking for my front speakers and subwoofer to be a little louder. Can I get by with spending all my money on a very good reciever (Denon, Marantz, Onkyo, etc.) and then connect it to the speakers that came with my current system, go with another HT in a box?
PSI'm looking to get 1 on the following recievers to connect to the speakers that I currently have (until I get some better speakers of course)
Pioneer VSX-920-K (I don't know when this model will be released)
I would put the VSX-1020 on the list, but from what I see, the additions to the 1020 don't serve any purpose for me.