If big: EarMen Tr-amp @ $$250…that’s the best solution for the HD 600s in terms of power. Also ok is the ifi Audio nano bl @$200, but it is pushing the HD 600/650 to its limits.

If big and budget: the $150 ifi Audio hip dac is great, too. See my review.

If small: Audioquest Dragonfly Cobalt @$300 or Red @$200…difference is apparently in the natural degree of sound. I have the Cobalt and will receive the Red tomorrow. Not cheap.

If cheaper: Helm Audio Bolt @ $100 or EarMen Eagle @ $130…or the new Shanling UA2 @ $85 (to be released in April). Apparently, the Hidzs SD9 at @109 is very good and has lots of power. Check the review at audiosciencereviews.com …he lists the output powers of many dongles…and what you need is power.

DAC not fax!

What portable fax would you recommend?

Why would you do that?

My Sonata HD Pro does get warm after few minutes connected to a phone (I tried 2 phones). But it doesn’t get as warm when using on my desktop.

Do you find any sonic benefit to amplifying another amplifier?

I’ve actually tried “double amping” the predecessor of the Sonata HD Pro (the non pro version), by connecting it to a Fiio A3 amp, and I don’t find any sonic benefits, but the Fiio A3 amp has a volume knob, so it is useful (cause the Sonata HD non pro hasn’t a volume control).

I have purchased the Pro version after the stellar reviews by your good self and other reviewers, so am waiting to get it! The non pro version wasn’t that bad, I liked it so much I bought 2 sets, but they grew hot with prolonged use, had an audible clicking sometimes when changing songs and of course, no volume controls.

Double amping principally works. For example, my iPhone’s headphone jack int my FiiO E12 Montblanc constitutes double amping.

The Sonata HD Pro already has an amplifier inside (in addition to a DAC), so it doesn’t require an additional external amp.

Doing so would be “double amping”, which while unlikely to damage any equipment, I’m not sure if it might cause clipping the in 2nd amp if the input voltage is too high. Maybe some electronic experts can advise?