1. Yes. There's a LARGE difference. Even with M50s, there's a large difference between those and HD25s. You can for sure tell the difference between the HD25 and Beats / SK / Bose / etc. Note that M50s were my first, HD25s second (for portability at least). I compared M50s side-by-side w/ all 3 versions of the Beats at Bestbuy and the one that sounded the closest were the.. metal ones (Pros?) and even then M50s sounded better.
2. No. Just ~$80 (depending where you buy it) more for looks.
3. Probably yes. Looking at the portable thread, top two were M50s and HD25s, and I first chose the M50s, and now have the HD25s and loving it
. M50s were great too, but were a bit of a hassle to use in public cause of the large size + made my ears sweat.
About the bass: IMO, I could not hear M50's bass at all w/o boosting it w/ an amp (using an E5 amp only too). Was way too small for me.
HD25 is largely present, but I have no idea what the terms are. I do know for sure that M50s can create lower frequencies far better than HD25s though. Talking about the deep rumbles (like 20hz~40hz or something, I dont know).
About HD25s needing an amp: IMO, they don't. I haven't tried a like 200+ portable amp yet, but they already sound better to me than M50s w/o an amp. Plus w/ an amp it just makes the portability more inconvenient. Using an E5 only, it was annoying as hell taking my phone in and out of my pocket and I don't even bother to use it anymore. Using an E7, didn't even bother to take my phone out and just left it on the same track. IMO amps are only for like the hardcore people as I'm pretty sure many new comers won't notice the difference well enough to justify the price. Then again I've never used a portable amp besides an E7 which apparently isn't much better than an E5
. Bass wise, no idea for amps.