Wait … why did the dongle not work? It should work, you just have to use the adapter that comes with HD600 to plug into the dongle.

Spotify free sounds whack. Paid version should sound just as good, but you need to change some settings in Spotify app to set quality to max (to use highest bit rate) and remove volume normalization (which adds compression artifacts).

Wired headphones, mate. Are you looking for something more specific?

Shpongle, Younger Brother, Ott, Shulman, Zero Cult, Entheogenic. Start with earlier albums and follow the artist as they evolve in time, each of them has excellent music and excellent production.

Sennheiser HD600, but it took a few tries. Unironically, I thought it sounded too bassy first time I heard it.

Basically scoop out the mids, boost bass below 100 hz to the skies, and throw in treble spikes to offset any opinion if smoothness. The worst variation of this idea I heard is Ultrasone, makes Beyerdynamic sound comparatively mellow. Westone also can sound like this, with a specific suck out right where HD600 has its upper mid presence spike.

Spotify - free tier - no. Paid tier - yes. Make sure to turn off volume normalization and set quality to highest in the settings, as it is the main reason a lot of people think it sounds bad.