Shopping...But, I Have Questions!

[KiterSuperFly]

I am considering getting a pair of over-ear headphones. My problem is that, since COVID, I have jumped down the audio rabbit hole, and what, in the past, would have been 'these fit nice and have good reviews' has turned into something more complicated.

My use will be in home. Source will iPad or iPhone. Although plugging into Denon AVR is an option on occasion. Music will be electronica/trippy alternative, so full low end is a must. Trying to keep it under ~$400USD, I am under the impression you can get a decent set for that. I'm big on finding good bang for my buck.

1. Does an iPhone or iPad function well in this situation, or, should one really consider an external DAC/AMP?
2. If the DAC/AMP is a must, am I correct in that I can plug Apple device directly into it from Lighting or USB-C into the DAC USB, than also get Hi-Res when available? Any other advantages to an External DAC/AMP?
3. Do any headphones have internal DAC's? Is that a thing? i.e.: plug them directly into iOS device for Hi-Res decoding?
4. PEQ. Can I EQ with an Apple device? If not, what are the options here? Do some headphones come with an iOS in-app PEQ then?
5. Spatial Audio over headphones has proven hit or miss, but any considerations there? I read it is better and actually kinda works with open back headphones.
6. Open back headphones seem to be recommended, but I have never used some. How loud is it outside them, and how much outside will I hear in them? (I guess this might be an "If you have to ask, get closed backs" question, now that I am typing it...
7. I see some headphones boasting frequency response down into single digits. how important is sub-20hz in headphones? I do listen to music with very deep bass...
8. I probably won't be taking these out of the house, but wired/wireless, ANR or not, are also questions.
9. If wireless, Codec support seems to be a consideration. As an Apple as the source user, Bluetooth 5 (or 5.1?) and AAC, but is there any other spec. that is a 'must have'?
10. I have read there may be a new wireless Codec coming that supports higher res? Maybe a good time to wait?

I am shocked by the number of models out there. hard to narrow down. Just looking for some broad strokes/pointers so I don't miss something obvious.

Thanks in advance.

 

[KiterSuperFly]

Couple more thoughts...
I'd bet that within the next year, some newly released iOS devices will support Hi-Res internally, no external DAC required. And, how important is Hi-Res anyway?

 

[jinjuku]

Do you want blue tooth options?

Current Apple products don't have headphone out so you will need a DAC. Are you seeking this for portable use?

I use the Hidiz S9 USB 3.0 portable DAC. It has balanced output. It's $99 for the DAC and $50 for a balanced TRRS headphone cable.

This one, from an instrumented view point, measures extremely well, and has good voltage output to drive low impedance HP's.

So the above is what I'm using with my HiFi Man HE4XX open back planar headphones. Absolutely love the combo. It's a potent, bang for the buck since I also picked up a balanced TRRS to XLR cable and can also use it as a desktop amp. Not bad for $200 all in.