I have Hifiman HE-4XX and HE-400i, and compared these 2 headphones. (Using DAC is RME ADI-2 DAC, Sound source is FLAC). These headphones have different sound quality trends, I felt that HE-4XX was more advanced especially in the bass. However if using with MP3 files, I felt HE-400i was good choice, because some badness of HE-4XX is very noticeable when the sound is not so good.
TK1103MKII4xx is a cheaper (bad word, economic?) model of the 400i for massdrop. They saved on headband material and such. The drivers are the same. The fit may not be. So this could lead to a slightly different sound. Additionally, these headphones need a good bit of time to break in. The bass will change over time. They don't "need" an amp but do benefit from one as well.
Ive told friends about the 4xx saying they were a great deal to get those drivers at the price. Now these are an incredible deal. I know it's simply due to HIFIMAN having newer models, but still. Really good cans!
TK1103MKIIThen if you haven't let them play long enough, it may help. Otherwise maybe the difference in pads? I don't know, I love the bass on the he400i. They aren't bass canons, but they're very detailed.
Oh. I think I know maybe why in a review I was reading comparing a few HIFIMAN models. There were a few versions of the he400i. One (circa 2014?) had a backplate and that amplified the sub-bass a bit. Though I ordered mine I think in like 2017. I read further and saw a revision in 2020 that seemed to take a turn for the worse unfortunately (why can't they just give things new numbers when changed? LoL, it's impossible to discuss and compare so we all end up with 12 pairs of headphones). Now out of production, I think I understand the purpose of the Sundaras now.
I also think hifimans are pretty friendly for some slight EQ without losing much if anything unless you really make major adjustments.
hypnoticdcimeThe planars driver headphones, my recommend is Fostex T60RP. If it's more expensive, Acoustic Research AR-H1.
I compared the old model of Hifiman HE-400i and new HE-4XX, after that I bought HE-5XX (from drop) and I like it !