Any technical advantages using USB C over 3.5 jack?

Yes. You bought the fiio K9 already, why wouldn't you use it?

Yes, the benefit of the k9 is better DAC than in the Mac (hence sending the pure digital file to the k9 via USB-C instead) and better amplification than the Mac.

I have a Mac mini myself and run a Ferrum Erco DAC + Amp via USB-C.

with my ears

Not having a MacBook.

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If I had a dac/amp of course i'd just use that - why would i buy it if I wasnt gonna use it?

MB’s headphone jack is adequate unless it just lives on a desk. I’m not really fond of lugging around a mess of dongles I need to set up and tear down for some absolutely marginal and most likely imaginary increase in audio quality

ii use dac/amp for other things like on a pc for gaming, Apple TV.

On macbook, it's portable. That Fiio is too heavy to move around.

yeah, it all seem to sound the same to me....but I'm not a trained audiophile to pick up sound nuances. I do notice the sound is different with Bluetooth, as expected.

I just use my macbook all the time with my Audeze at work. Works pretty fine. Hardly worth dragging dongles or dacs to work.

i meant in the house. that Dac Amp is not portable.

I have an Aune DAC and Sabaj amp at home and do use those rather than the onboard MacBook one. Might be placebo though ..

Ah OK i understand. It's up to you tbh, just try with/without.

In my case I run a Edition XS and I can definitely hear a very tangible and significant difference - but I think? am not sure? the HD800S is easier to drive? So you can just test it out and decide for yourself whether the portability benefit is worth the difference in SQ

Perfect thanks

This is too specific for anyone to provide a meaningful answer.

You'd have to try both tbh to get a concrete answer.

If i was using a HD800, tho, Id go external OTL tube amp. Not one of your options, but really is a thoroughly enjoyable (and not particularly necessary) alternative/option.