Well I found the 681 evo too uncomfortable for me, I'm at the computer for 6+ hours at a time so I need something super comfy. The evo is great at its price but something less comfortable was not a option. The 558s fill my needs but obviously are 4x the price.
Oh, 669 is closed back, thats why I never heard/looked into it.
Amazon reviews (and any review on a retail site) are worthless, as you can see its highly rated but so is any headphone, thats just because 99% of people are users, NOT reviewers or enthusiasts. Meaning 99% of people dont use them in strict conditions, and they buy a single set of headphones, CPUs, power tools, whatever the item in question is, every 3-5 years, if that.
They're listening to youtube or itunes mp3s off ipods or cell phones, and dont knpw what good sound is. They compare them to the airplane headphones or stock ipod headphones, not the superlux evo or monoprice 8323, etc. So everyone gives 5 stars. Then, the 1 star reviews are always idiots, people saying things like it doesnt come with a mic, the cable isnt usb, etc. Seriously, read the negative reviews on it. Even 558s, the majority of 1-star reviews are 'it leaks sound' (derp its open).
Im not saying anything about the m40fs, Ive never heard of it (seeing as I dont care for leakage as these are literally to replace my pc speakers, I only researched open phones), but amazon, newegg, bestbuy, etc reviews mean nothing. Even beats are 5-star.
Nice list beocord, I'd like to try out the takstars one day. Never heard of the pro monitor.
All the superluxes are great under $50. I see it less as evo vs 681 and more like choose which variation you like more. Remember, the evo is only $38 while the others are a few bucks more, and not to mention the sligtly increased comfort, detachable cable, and build/aesthetics, I think it makes all of them pretty even. The sound quality isnt that different between the models, its mostly preference between the models imo.
Though I've never heard anyone say 681 over 681 evo, I hear other way around more. Or that the 681B is the best.