Hi-
 
I think you should invest in a slightly better setup. I owned these, and used them with an Oppo HA-2 on high gain, and they were one of the best all-around headphones I've ever heard. 
 
I also had an HTC One M8, and its audio quality is passible at best. I dont' think it's fair to use that source with the HE-400i, a headphone that retails for $500, and designed to pair with higher-end audio equipment. 
 
The Hifiman HE-400s, which retails for $300, is similar to the HE-400i, will probably be a better match for your gear. 
 
In summary, I think a 3.5-star review of this headphone when purchased at $300 is unfair. 
 
Just my two cents.

Hi,
an interesting review in a sense you shared your experience honestly and tried to describe pros/cons of  HE400i. Also photos and graphs are all right. On the other side I also think HE-400i should have been reviewed with a better rig than HTC phone or laptop. DACs in laptops are usually particularly weak. (poor bass extension, shrill treble, thin midrange)  From my experience HTC phones have also long way to be considered decent audio source to put it mildly.
Bottom line: A promising review but would be nicely"extended" if other (better) audio setups had been included.

Thanks for sharing your impressions from your "not sure what to expect" point of view.... I do agree with the first comment that the HE-400s, which are $299, might be a better match for your gear.

Yes, the 400i is not really easy to drive. It isn't a killer either, but I do tend to agree with the others who have commented that your source was quite likely a big part of the issues you experienced with the 400i. I do sympathize as it can be expensive to set-up a rig, and you started with arguably the most important part of the chain, a decent headphone. If you can find a way through selling other gear and saving something like the Massdrop Grace M9XX would be an excellent all in one solution as would a JDS Elements.
 
Anyway, good honest review and don't be discouraged, I'm glad you took the risk and reviewed the 400i. It isn't easy to publically take a risk so good for you. 

Nice honest review from your point of view with the gear you have.