Hi, I'm new to Head-fi. Also this is the first time I actually ever post on websites, so if I'm a bit slow, just let me know!
anyways, as the title says, I'm looking for open-back headphones and right now I got my eyes set on the Hifiman HE-400i atm.
I really want some, but I'm trying to find a great price for them because I just got my ath-msr7's...
Any advice about anything is gladly accepted.
Also anyone know differences between the ath-msr7 and HE-400i? Ik the ath's are closed back and such.

Make sure you have enough current for planars like the 400i. Amp is important for planars. A lot of people are satisfied with their schiit M/M stack. 400i is a great pair btw, better if you can find a secondhand that has been purchased when it was $300 new. Sennheiser 598, 600, 650 beyerdynamic dt880, 990, t90 fostex th600 thx00 akg 701/q701 are all great cans for starters and even endgame for a lot.

I am sorry to say I can compare the HE-400i with the ath-msr7, having not heard the latter. If you want a good price then you may be able to find them used here. Also since Black Friday they have gone on sale several times for $299, so patience may be your friend here if you wanna go new.
 
The HE-400i used to be my go to for music. The only reason I would take off my Sennheiser HD 600 was for them. I really enjoyed the bass response and how great vocals, both female and male, were presented. My only disappointment was that despite being open back, the sound-stage was actually quite small, so if you are interested in open backs for this reason you may want to experiment with another can like the AKG K7XX or the like.
 
I was driving them through my Schiit Magni/Modi stack, from what I remember they weren't that easy to drive so like willowbrook said they may need some love in this area.

I actually have a DAC coming in today. I got the FiiO E10 which I heard is really good.
How would those work? Also I did look into the akg k712 pro.

How do the magni/modi sound?

I actually have looked into the dt880 pro's. Almost bought them a few days ago actually. How do they typically sound? Do they need an amp?

IMO the Magni/Modi is a price-per-performance champion. It does not color the sound at all, to my ear, and it will drive pretty much anything you can throw at it. Headphones that I have run through this without problem: AKG K7xx, Sennheiser HD 600, Hifiman HE-400i, modded Fostex T50RP, and Audeze LCD-2. Another popular budget combo is O2+ODAC.
 
Solid state is usually a pretty solid starting place, although if you are adventurous you can research tube amps.
 
As for the DT 880's they would benefit from an amp and would probably have trouble off a motherboard, phone, etc. I've heard good things about the 880s being more neutral than it's brothers the 770s and 990s, but I think the Beyer treble is still present (This is just what I've heard from others, there is quite a bit of discussion about this phone here and around the internet).
 
I had the Fiio E17K, it did a decent job with my 400i, but I did notice a change for the better when using the Schiit stack. The Fiio did work well with my DT 770 32 ohm and AKG K7xx though.

I've had DT880, sounds pretty good. However, for me bass and midrange was lacking while it was a tad bit too bright for me. I'm not a total treblehead, but I do love my treble. Just not with the NFB-11 I tried with. That's for me and others may feel different. Which is why you need to start with something and work your way there. Best is to demo in shops to find what you want, but if you can't I suggest grabbing something neutral like the HD600 and see if you like it or not. It can be a reference point of whether you need or don't need some of this and that. Tons of information online to help you.

HD598 and fiio is really good for entry too.

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Mobile headfi is messed up. Im sorry.

Oh no its fine! and thanks for the input. Do you think jumping to the HE-400i immediately is too fast? I have been looking at the HD600 though.

Not at all, anything is a good starting point as long as you can leave the budget for amp/DAC too. Schiit stack should do well with the 400i.