The reason I ask: I'm looking to purchase one or the other. The extra price for the 500s is acceptable to me if there -is- some [even minor] difference in sound quality. The volume control is of absolutely no importance to me.
Moral: If you're all really convinced that the technical specs of the 400's and 500's are the same [in spite of Sennheiser's claim] - then I'll save a few bucks and go with the 400's.
Unfortunately the store won't let me demo either pair.
If anything, the MX400 would sound better because the in-line volume might degrade sound.
Don't worry, they're absolutely the same, the frequency response is just a marketing thing and is, btw, fully neglectable.
For the $12 that MX400's go for, just buy a pair and enjoy them for the utilitarian, throw-in-your-bag-everyday, earbuds they are. Or if only the MX500's are available to you for some reason, get those for a few $$ more. Don't fret. Either are worth way more than the enjoyment you'll get out of them.
Just wanted to say (a belated) thanks to the both of you for the advice. I ended up going with the MX400's and am quite pleased with them given their very respectable price.
In case anyone else ends up reading this and is interested, I can't compare them to any competing earbuds as such, but I can recommend them based on their own merits (again, particularly re: the price).