Oh I get what you mean, my little brother went on a headphone buying spree, and if they cost 200$ to about 1000$ he bought them, granted ebay prices, but he still bought them. The only real area my 598's are lacking is bass that has a punch. doing tests to see what range I can hear sound in, still puts me above average, knowing sennheiser isn't the best for the top range, and having ones on my head that are suppose to be the best in that range, there is VERY little difference, so I stopped looking at that area, and started looking more at the bass. His fastex are fantastic for bass, but I don't have an amp that can properly drive then, along with when using his amp they were a very piercing high end to the point that just wasn't comfortable. His hifiman's he got that were 1000$ ones, they were better than what I have in all regards, but the price he paid for them was not better by enough to ever justify it.
I'm looking at these knowing monoprice as a company, they put out things that I have gotten that would have costed more if I had gotten them elsewhere while performing as good or better than an alternative product. This has me at least interested in their headphones and I'm watching for reviews from people who have more experience then I do to trickel in, possibly when my brother inevitably spends some money on a pair for himself. When compared to the hifiman you said, the he-400i if everything is the same, as in both headphones are equal and the only difference was how they look, I would be willing to go monoprice because I much prefer how it looks and its 50$ more. When I got my 598's I was looking at the 558's, but 2 things made me go 598. First, they acknowledged a cracking problem, were actively replacing everyone that was cracked, and for all new ones used a different blend of or just different plastic. It's been 2 or 3 years now, and they look like new beside the earpads and headband that I replaced recently. And 2 being the look, but I'm apparently weird in that way where I like the look of noctua fans for pc, not just begrudgingly use them because they are the best performing fans on the market.
As for why keep up now when a purchase is a long time down the line baring outstanding performance... Have you ever bought something, didn't need a new one, then one day down the line it breaks, it's out of warranty, you need a new one right now, but its been years and you need to sift through a crap ton if new advancements and new models? Had it happen twice to me on computers, I didn't know much about power supplies, had one blow up in my case, that was a nerve wreaking 2 days thinking my entire computer was gone as the powersupply took everything with it. Thankfully the cheap piece of crap had either a fuse or some kind of breaker inside to cut all power to the computer before it blew and nothing was damaged. Now I know there are really only 3 brands that are worth a damn, corsair as they use seasonic designs, seasonic, and superflower as superflower are I believe the best quality, but they don't make many low end models that aren't rebranded for other companies. The second was the 1.5tb hdds, I bought one because I was doing a fresh format and I always get a new one so I can't lose old data when I format, so I went to best buy, saw 1.5tb, and saw 140$... ok, that's my price range. two months later my pc died due to age, so I got another 1.5tb, this time for 90$ as it was clearance. So, one drive just started having a crap ton of errors and the boot was click of deathing, thankfully no data loss, was able to mirror it before it gave up the ghost. Replace them with rma drives, one is ok, the other literally vibrated itself to death. and a few months back the other stopped working completely. I now know that every company has horrific 1.5tb drives, but seagate, the brand I bought had a 50%+ fail rate on the drives. So I keep up with company failure rates, and seagate is still one of the best especially for high capacity drives, with hitachi leading the pack but also commanding a larger price. Had crap like this happen to me allot where something that I require breaks and I need to get a new one, but not knowing anything about it screwed me. So now I make a point to keep up with everything I use. And as I learned a LONG time ago, when it comes to audio, nothing really changes, a newer model does not make the old one worthless, but because of the newer model people get rid of fantastic equipment for very low prices. If I had to choose right now what to get, I would likely go with an ebay hd650, possibly a hd700 if I could find it for 300 or less, and order some replacement pads for it because I know the sennheiser style and know it would be both an upgrade for me, and fit very comfortably, but that monoprice has a 106 mm driver, and thats bigger than the entire can of my hd598's so its possibly an ever more comfortable wear for me, as I disqualify anything that touches my ears instantly from useable, I have never been able to find that comfortable.
After getting some kz3 iems, I would honestly be hardpressed to recommend monks, as for 5$ more you trade off a minimal amount of treble and mid for decent bass, for less then 10$
you want to spend 100$ on portable headphones, iems are the only thing to consider, get a pair of monks for emergencies/backups and get some nice iems for everything else.
for actual headphones, its hard to recommend anything over 200$, as I have been exposed to a shitload of them thanks to my little brother buying everything under the sun because 'headphones are my hobby' When I have properly amped up 1000$ flagships on my head, and all I can say is 'yea, they are the best, but 800$ more then mine good but they are better'
In all honesty, Im looking at the Monolith M1060 Planar Headphones from monoprice right now as depending on what they put out, it could very well be the definitive headphone for that price range, can't wait till people I trust get their hands on it as i am looking for something that can handle bass a bit better and these look to be across the board better then what I have.