Hey JediMa70, I replied to your post on ASR, where they also measured & reviewed the SMSP SP200. The SP200 has impressive measurements and looks like a great amp. But remember, most amps are measured at or near full-scale output, which nobody actually uses because it's way too loud for listening. And, like most amps of conventional design (volume upstream from fixed gain ratio), as you turn down the volume, the SP200 measures worse. The Corda Jazz outperforms the SP200 at 50 mV, with a significantly better SNR and perfect channel balance at all volume settings. It also has crossfeed, which the SP200 doesn't have.
I'm a big believer in measurements, but you've got to measure correctly, understand the limitations, and how they relate to actual listening conditions. I wrote more on this subject
here.
Generally speaking, there are many headphone amps with impressive SNR and SINAD, better on paper than the Jazz appears. But with these amps, as you turn down the volume to actual listening levels those measurements become less impressive. Amps that measure and sound as good as the Jazz at normal listening levels, are rare. This is what is unique about Jan Meier's design and philosophy. Where most companies engineer to maximize measurements on paper, Meier engineers to make the amp more transparent at the normal listening levels people actually use.
The only amp I know of that comes close to the Jazz, and is reasonably priced, is the JDS Atom. I have both (a Corda Jazz and an Atom) and have used them daily for over a year. The Jazz is built better than the Atom, and it has crossfeed. The Atom is a lot more powerful, but since the Jazz can take the HD-600 to about 116 dB SPL, you've got to question whether you would actually use that extra power. Perhaps you would if you had an unusually power hungry headphone like the HiFiMan HE-6. But for any normal headphone, I'd take the Jazz.
In short, since you already own the Jazz, my advice is to keep it. I don't think you can get a better -- by better I mean one that performs better at actual listening levels -- for under a kilobuck. One thing to consider is upgrading to FF, if it doesn't already have that.