My main receiver for a couple years has been a SX-650 and it's served pretty well despite the slight 'muffle' (too much low-mid) that I hear across all Pioneers I've owned (I 'fix it' with a 10 band Marantz EQ the best I can...). Today I picked up a SX-680 for $17 at the thrift (score!). I actually kinda anticipated it would be half-busted, but I just fired it up and all is well. Actually better than that...it sounds great!! What gets me is the model numbers being so close to each other, but the looks and sound being pretty radically different. The 680 sounds more 'scooped' (like my Sansui) and has less low-mid mud. I was going to clean it up, cap it, and re-sell to a friend, but I'm thinking I may keep it. So...looking for a little context: How can 2 receivers with such close model numbers look and sound so different? (obviously...because they're different....but how? are they different eras? radically different electronics? I would have expected a 650 and 680 to be almost the same with slightly different power ratings or something) Anyway...any enlightenment from the Pioneer nerds? Gracias, Amigos!