"Guns for show, knives for a pro..."
Living Room: B&W 804D, B&W CMC S2, B&W ASW610, Yamaha RX-A2030, Rotel RC-1570, Rotel RB-1582 mkii, Oppo BDP-93, ATV.
Office: KEF LS50s, B&W ASW608, Parasound P5, Parasound A23, Marantz CD5004, Rega RP1, Bluesound Node 2.
My above is the simple story as it's up to you to make my above as complicated as you want. My recommendation is to buy what ever makes you feel all warm and fuzzy inside.
From the spec page, the 685 goes down to 49hz while the 684 only goes down another 4hz to 45hz, so you would likely be better served with the 684s and a dedicated subwoofer.
Frequency range -6dB at 34Hz and 50kHz
Frequency response 44Hz - 22kHz ±3dB on reference
Put simply, the lowest 12 notes (or thereabout, out of 88 total) of the piano sound reproduced from the speakers fall off in volume and sound quieter than the rest, the lower the quieter. Depending on what type of music and what volume you listen at this may be sufficient without a sub.
You don't need a receiver to decode movie sound formats as modern BDPs all do that.
For a 2CH system you only 'need' a receiver if you need other features only found in a receiver and not in an integrated amp.
Audiosceptics accept audio trials using 25 people. A recent Oxford study with over 353,000 patient records from 639 separate clinical trials shows for every 1,000 people taking diclofenac or ibuprofen there would be 3 additional heart attacks, 4 more cases of heart failure and 1 death every year.