Marco, here are some preamps (used) I'd consider for your application at or below your price limit. None of them will be embarrassed by the pairing with an Atma-Sphere amp. They're all tube units, they're all single-ended (RCA), and none of them do anything wrong to the sound...
Conrad-Johnson PV10A or PV10B
Conrad-Johnson PV9
Ear 834L
Consonance T-1 or T-99 (the T-99 is only $825 new)
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Conrad-Johnson PV10A or PV10B
Conrad-Johnson PV9
Ear 834L
Consonance T-1 or T-99 (the T-99 is only $825 new)
.