I use my iPad for ear training and I can absolutely tell the difference between my DragonFly red and the Apple USB-C DAC. I'm just better with the DragonFly.
Don't get me started on the Bluetooth headsets. For music they are ok but there's really basic interval training that I score 100% on with the DAC and like 80% with Bluetooth. Consistently.
I use Complete Ear Training for interval training, with the grand piano or the Rhodes sound pack. I switch pack every once in a while. I also use TonalEnergy Tuner to sing intervals or scales to it and see if I'm in tune.
In something like a year I went from "telling apart major and minor thirds is black magic" to being able to sing the major scale right like 60% of the time and I have an 80% success rate for all intervals up to a fifth. It did improve my piano improvisational skills by a lot.
Don't get me started on the Bluetooth headsets. For music they are ok but there's really basic interval training that I score 100% on with the DAC and like 80% with Bluetooth. Consistently.