Where is the price sweet spot for a DAC/AMP?

For context, I own a DT990 pro (80ohm) I'm looking at getting a HD 6XX or hifiman edition XS and I was wondering where the sweet spot would be for the source. Something that would comfortably power all of the headphones mentioned maybe with some headroom for potential future purchases

I'm interested in the most transparent sound signature possible and a simple interface, no fancy bluetooth or anything like that. Plug-and-play USB support as I will be listening mostly at my computer but something portable would also be nice

I'm considering getting a portable dac/amp for cheap and using at my PC, something like the Moondrop Dawn pro (cheap, but maybe not enough power), Fiio KA13 and Fiio KA 17 or getting a dedicated schiit magni unity with a dac
I don't know how much power is really necessary, do the dacs really sound much different?
I'd like to buy a Fiio KA13 and use that for both desktop and portable, but will it deliver enough power to last me for many years? Is the sound really transparent?

I don't have too much money so I'd rather buy something cheap but also one that will last me as long as possible. I'd hate to spend money on differences i cant hear

As a side note, I'm comfortable with rewiring my headphones for balanced audio if the normal outputs don't have enough power
As another side note, I'm currently running a blue yeti as my dac/amp (far from great i know), does anyone know how much power it delivers and how good the dac is? Would the headphone output from a Focusrite scarlett 2i2 be good for my needs? (may as well kill two birds with one stone if i can :)

All opinions or suggestions are welcome!