What if my source doesn’t support aptX? Can I still use HD6 and if so, how will it sound?

You can absolutely still use the HD6 and it will sound great! AptX is a high-quality Bluetooth audio codec that works really well. In addition to mandatory support for SBC, Bluetooth also includes optional support for many other codecs, like MPEG-1, MPEG-2, MPEG-4, and AAC, all of which have their own advantages, and some of which sound very close to AptX.