Yeah but the tweeter is the same size. I dunno. I wouldn't have expected that.
Other subtle differences that are probably my imagination:
* The sixes replay so effortlessly as I increased or decreased volume - i.e. smooth dynamic transitions. The five feels a little restrained as if it's working harder, which it is. This goes away once the new level is reached.
* The fives change the soundstage more when the acoustic axis is moved or tilted. I find the sixes harder to place.
* Fives did not like a lower crossover. They flashed clipping very quickly. Generally 100hz works best for my square room so it's not an issue but get a big subwoofer if you are going with the 8351b.
* The 8361a at twice the weight (70 vs 40 lbs) are more stable on the pedestal stands I use. The fives move more but are still safe for my use, but not good with kids or pets.
* This has been covered before but the fives (without subs) can just barely hit 85db of bass heavy music at 1.8m in a 10x10 room. Alone, the sixes add at least 10db when listening to bass heavy content. I would not buy the 8351b for a big room.
* I liked the performance of both nearfield (inside 2m) and a smaller room than midfield in a larger room. I found myself missing wideer directivity despite the reported coaxial ability to hold the image off axis.
It goes without saying that both of these are endgame when paired with a subwoofer. There is much more similarity than difference.