Honestly the more important part in a car audio system is the processor and not the file format, because at least that deals with the fact that you don't drive a Mclaren F1 or a fully customized ///////ALPINE show car (ie, center drive, equidistant to all speakers, which is how you sit at home or how you wear headphones). So as long you have a way to play 320kbps MP3 off a thumb drive or something with the stock receiver (which I assume you don't want to remove, considering you're considering a DAP rather than an adapter to fit an ///////ALPINE 2-DIN with optical output, or the matching processor that uses Ai-Net for control and digital audio transmission), I'd really worry a lot more with having time alignment, crossovers, and a parametric EQ than being able to use high res files without conversion.
If not, or if you really want to use high res files as such, I'd explore other options than a DAP, more so since you're spending on a processor like that anyway (and I assume custom molded speaker mounts). Like, rip out the stock system, use an adapter that mounts a small Android tablet on the dash in place of the receiver, but instead of using that as the music player (ie some people have mounted tablets to use Spotify as well as local storage, then sending out digital audio to a DAC, then to an analogue input ADC-DSP-DAC integration processor), use it just as an interface for an Android miniPC like the Minix Neo. Hide that near the Audison amp so you can use a short toslink cable that is neatly tucked in and doesn't move so it doesn't break easily, plus a solid state drive in a USB external enclosure (you could probably put this in a more accessible area).
Windows 10 Intel-powered NUCs have one internal 2.5in drive bay plus some have a spare M.2 slot, but I'm still more into the Android miniPC since 1) you don't need the processing power of an Intel Core processor and 2) it's still better if you can more easily pull the drive out and use it between the two cars each with their own miniPC (or have two drives, whichever, at least it's still easier to take it out and add to the contents).
I already know guys using these is why I ask and they are loved apparently. Believe me I have been through every idea you have mentioned and have been into car audio and competing in IASCA, MECA and USACi comps for years. I am big into Sound quality and was hooked in competition when I got my first place in IASCA.
I planned to use a magnet mount to hold it in place like I use for my iphone currently and a cable sticking out is by no means a deal breaker. The other options you mentioned are far more expensive and would have to be done in both cars to stay in place. My background is computers and servers so I am very well aware of the Car PC route. I understand how to make the car sound good but the source unit/material is still one of THE most important items in an SQ system.
I have the rest of my system done and full digital Audison. The Alpine double din's don't play high resolution audio and only one of my cars has a double din location.. if I was going that route I would run the new Sony in each car and spend 2k$.. or I can use a DAP and get the same results for 1/4 of the price.
The Androild Tablet and Ipad options are common as well but bulky. My Daily car is a compact piece of sh!t lol.. the good thing is no one knows there is system in the car. My other car is a Supercharged Mustang that draws a little attention. Keeping the stock headunit in place connected to my Audison Bit one and having the DAP option will at least appear stock. I really want to take the path of the DAP as it truly saves me a ton of money and can be used for so much more. My question is more or less which DAP to get. Amplified power is no concern to me as 95% of the time it will be using optical. Storage, file play back, durability, battery life, the ability to stream would be nice as well.
Is there an adapter for an iphone that provides optical output? I would love to stream Tidal over the same optical cable I had planned to use for the DAP... just swap out as needed. Having to worry about a cable is no big deal and I can use a split up front so if i break it off I just have to replace a short piece and not the whole long run back to the processor (like you mentioned. It's ashame the Audison Bit play HD is such a flop.
Keep in mind my whole setup will be digital up until the amp it's self before it goes analog. Optical from source to DSP, digital cat5e from DSP to AMP using Audison FULL DA and then the amp's DAC will convert and toss out the signal to the speakers. Doing anything analog at the start of this chain would ruin the point of going all digital.
Thank you for the reply! You seem to have some car audio knowledge so that helps me A LOT!