This will be unpopular opinion at Reddit since everyone here think loud enough is loud enough.

But from my experience driving Mad Dog, I own Dragonfly Red (dongle), O2/SDAC, Violectric V200. Mad Dog sounded the best on O2/SDAC in my opinion.

People might argue it's all in your head because you anticipate more cost = better. Well V200 cost the most and it's not the best pairing in my opinion.

People might argue well maybe you like this DAC/amp more so you think it sound better, well my favourite dac/amp is Dragonfly Red for a longgggggggg time and it sounded absolutely TERRIBLE with Mad Dog. Same experience with LCD-2C where Dragonfly Red is just terrible. But on most Denon/Fostex headphone I just LOVE the Dragonfly Red.

So I would actually say K3 is most likely not enough to power it properly similar to Dragonfly Red since both are USB powered. Maybe try a FiiO K7 which is currently one of the better price per performance dac/amp.

Again I based this on my listening experience with no guarantee your experience will be the same, but this is my best guess.

EDIT: Oh and try all the gain setting, quite often low gain at the same volume actually doesn't sound the same at high gain same volume. In theory it should be the same but in actuality it doesn't.