I doubt you're going to find someone who has compared both in a variety of systems over a lengthy period of time - even top-notch preamps generally have a subtle effect on a system and take months to properly evaluate. I would draw out any person who directly responds to your question about the other components in their system, their listening room, whether they have treated their A/C power supply, the type of music they listen to, etc.
At this level, both of these preamps are going to be superb, and synergy and features should be your focus. If you have long IC runs (2+ meters) to your amp, I would be inclined to consider the top solid-state pre's in order to avoid the impedance issues and problems running long cables that almost all tube preamps (except those using output transformers) suffer from. I would insist upon a preamp that has a polarity switch, and perhaps one that allows you to set input levels independently for each input so as to avoid gross differences in base volume levels among various source components. I don't much like preamps with tube rectification (I think the Rex; not sure about the Atma-Sphere) because I can't keep them turned on 24/7.
I would add to your list of reference-level, truly balanced preamps the Rowland Criterion, ARC's recent-production reference preamps, and Pass's top preamp. The darTZeel preamp is not balanced (Herve Deletraz hates balanced circuits), but it does feature an XLR output on an output transformer, which confers the noise-suppresion benefits of balanced operation for running long IC's. I run a Rowland Coherence II (which is fully differential balanced and battery-powered).
At this level, both of these preamps are going to be superb, and synergy and features should be your focus. If you have long IC runs (2+ meters) to your amp, I would be inclined to consider the top solid-state pre's in order to avoid the impedance issues and problems running long cables that almost all tube preamps (except those using output transformers) suffer from. I would insist upon a preamp that has a polarity switch, and perhaps one that allows you to set input levels independently for each input so as to avoid gross differences in base volume levels among various source components. I don't much like preamps with tube rectification (I think the Rex; not sure about the Atma-Sphere) because I can't keep them turned on 24/7.
I would add to your list of reference-level, truly balanced preamps the Rowland Criterion, ARC's recent-production reference preamps, and Pass's top preamp. The darTZeel preamp is not balanced (Herve Deletraz hates balanced circuits), but it does feature an XLR output on an output transformer, which confers the noise-suppresion benefits of balanced operation for running long IC's. I run a Rowland Coherence II (which is fully differential balanced and battery-powered).