Just got back, out of town for a few hours today.
I have not experimented with the Ultra yet, just been too busy, when I get more time and get more gear out of storage I will try the different outputs and see what I think is best but so far from several posts it seems the optical and using DDC might be best, I hope not to need the DDC but if it does improve things that is a very low cost upgrade
These days and in some vehicles in the past stock car systems can be quite good and also difficult to upgrade at a worthwhile cost. My beloved 2003 V8 AWD 4runner with 315k miles has the stock JBL system in it and sounds pretty dang good, just needs a bit more bass and I would not change anything else except I keep putting off a new HU as mine does not have NAV.
I have also thrown away many brand new Bose car audio systems as all I have dealt with were just crap, maybe some of the newer ones are better but not holding my breath. I developed a complete audio and sound deadening package for the C5 and C6 Vette and sold a bunch of them, only added 40 lbs and made the car not only made the audio hugely better, lower noise and lower cabin heat.
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I do agree, psycho-auditory system, we are all inherently different and constantly changing due to epigenetics, a facinating field of study.
I find it very easy to accept we might just hear things differently and or things others might or might not hear and the current measurement methods and gear just might not cover everything possible, more to come I am sure.
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I am not familiar with your whole system so only can comment on what I am familiar with or would estimate things to be true.
A couple of things to consider.
In the analog realm we want the highest, none distortion, signal starting from the source and feeding through each gear as we can dial in, it is called setting up the gain stage.
In digital whatever is the highest clean bit rate one can pass from one section to the next and then when in analog set the gains stage.
Cables matter but one can build very low cost great cables that are plenty good enough so make sure those are taken care of. Digital cables, Ethernet, etc, just decent cables are enough for most systems, if a very fine one perhaps, a bit higher end cables might be needed.
Great power supplies car do wonders and not cost a fortune.
Clean power from the wall is nice to have, my setup has two dedicated 20 amp, dual shielded power runs from the panel to the system outlets where I used hospital grade outlets for tight fitment. I do not believe in spending more money on "audiophile outlets" though there might be some merit in some that are not insanely overpriced.
Since there are a bunch of smart meters very close to my system I brought in the shielded power and am shielding the area really well but soon will move our RV to a spot without any meters near by.
Ethernet to the Ultra is the only way I would run it, I not use wifi nor use BT when I can avoid it. I might be a bit paranoid in this area but I spend many years withing feet of power source and weapons I cannot say more about.
I will look over the whole system but for now your front end up to the DAC is a very nice setup, I am considering the same exact DAC and do some chip rolling for fun and hopefully land on one I really like
Not sure about your sub, back in my car audio comp days I had JB subs for a very short time, sounded fun but in the end did not like their "signature sound" as it was not accurate be any means. Might not be true with the one you have.
Going to take a look into the amp, speakers, etc now.
Have a great evening!
Rick