Hi. No problem. Glad I can help. Do remember that these are just my own personal opinions. The 750dj and the m50 weighs about the same and are about the same size as they're both full sized headphones. If anything, the m50 might even be a "bit" heavier because of the type of plastic it uses. Contrary to what the pics look like, the 750dj's are made from plastic as well and it's a lighter, maybe even flimsier plastic compared with the m50 and hd25. When holding all 3, it feels like the plastic used for the m50 and hd25 are of higher quality. both of these headphones feels like they're constructed very well. The 750dj just feels like it's a tad flimsier in both construction and material. However, I have never had any build issues with the 750dj or with any of the other 2 headphones. I do take good care of my gear though. So overall if you take good care of your stuff it should be fine.
I can't really comment on durability because all my headphones are still working just fine. The 750dj though comes with more accessories than the m50. 750dj's have an extra pair of ear pads and it's cable is detachable, which is something I like since if the cable ever breaks all you have to do is replace it instead of buying a new pair of cans. The cable on the m50 is attached to it.
As for fit and comfort. The m50's are the most comfortable out of the 3. The hd25 is light and fits well, but it clamps pretty hard, which is a good and bad thing. It's clamping force might irritate a bit with extended use, but it also isolates outside noise very well as a result. The 750dj ear cups are round, which makes them an odd fit for an over the ear phone. Because of this, parts of the ear cup will actually be "on" your ear instead of over the whole thing. I don't find this uncomfortable at all and it still stays on my head fine; just it won't fit quite as snug as the other two. The m50's ear cup is oval so it completely goes over the ear, which makes them more comfortable and gives them a more steady fit and as a result of it being completely over the ear it isolates outside noise a bit better than the 750dj.
I've only demo'd a couple of grado's before for 15 minutes so I don't think I'm qualified to compare the grado's to these headphones. However, the 750dj's have good detail. The m50 on the other hand when first listening, you might feel like it's extremely detailed because of it's bass and treble boost. Coupled with it's recessed mids, you often notice more details in the treble for certain tracks. But a large part of this is due to the recessed mids, which takes you out of the music. The 750dj's on the other hand, once you match the volume, has just as much detail as the m50 if not more because of it's better mids. The sound of the 750dj's are more musical and it's a more immersive experience compared to the m50. The hd25's are "loud and clear" as I think that was the intention when they designed it since it's used for a lot of interviews and broadcasts, but I think perhaps it's drivers aren't as resolving as the other two headphones with their newer drivers. But after 25 odd years for the hd25 to still be one of the top "on ear" cans is a testament in itself.
Both the 750dj and m50 has a bigger soundstage than the hd25 to my ears, but this isn't saying much since the hd25 doesn't have much soundstage to begin with. haha.
So overall i'd say 750dj's sound the best hands down for me. They have a tighter bass, better mids and are overall more musical and immersive than the m50. And it's sound is less fatiguing and has a bigger sound stage than the hd25. I personally don't even have a problems with part of it's cup being on my ears instead of over it. My only complaint is how the corners of the headband kind of sticks out when you wear them. This makes it look like you've got some antenna's on your head. But if you don't mind that, they definately sound great.
The m50's on the other hand, with their boomy bass and recessed mids can be an issue for certain songs. For example when listening to certain Hendrix live tracks like say Little Wing live at Royal Albert Hall, Noel Redding's bass often overtakes Jimi's voice and guitars a bit. Or on certain Beatles tracks like "Something"; a track that already has recessed mids, Paul's bass would overtake George's singing even more. But on the 750dj's they sound much more balanced.
The HD25's since they are smaller 'on ear' cans are better for portability. the 750dj and m50's are both about the same size since they're both full sized cans.
The m50's i'd say are the most comfortable in terms of it's fit. And compared to the 750dj's, the m50 fits a bit better, which might be a consideration if you want to walk around while wearing them. But I've walked around with the 750dj's on my head just fine too.
I guess it just depends whether you want sound vs fit/comfort vs size/portability.
Good luck with the choice man. Interested to see what you choose and how you like them.
Cheers!