tigon_ridge
1000+ Head-Fier
Both are clean. Pretty much every headphone amp is clean. Any distortion that is audible is orders of magnitude more due to the headphones than the amps. A "magical" sounding amp is whichever pairs best with your headphone(s). My Schiit Vali 2+ doesn't sound all too magical with the Sundara, even after trying two other tubes. I have not tried other Schiit amps, besides the Asgard 2 and OG Mjolnir, neither of which has the warmth needed to pair well with the Sundara. Newer Schiits seem to be warmer, according to impressions. The Sundara seems to prefer warm sounding amps that are virtually unlimited by current output, i.e. speaker amps, via speaker taps. Now, those can be "dirty," with significant noise floor.With Topping, you know you're going to get a clean sounding device. With Schiit, you know you're going to get a magical sounding one. I choose magical, anytime. But it's not my purchase, so it's really up to the buyer, what they're looking for.
I think what many refer to as "clean" is actually just an overdamped sound. This is the type of sound that often feels thin, dry and lifeless, lacking naturalness and body/presence. True cleanliness is simply an output signal that is faithful to its input, and this is measurable. However, every modern headphone amp is capable of levels of cleanliness that are far beyond human detection, such that any "dirtiness" is imperceptible compared to a theoretical 0% THD + noise signal.
Two amps can have virtually identical distortion measurements and still sound quite different. These electrical signals outputed by different amps may measure identically, but behave differently when driving transducers, and this discrepancy even varies in degree by different headphones. The Sundara, for example, goes from "okay" sounding with my HP amps to completely transformed through my speaker taps. My HE-6se v2 doesn't seem to exhibit quite the same drastic change. It is intriguing.