My first suspicion would be the tube(s), but I assume you have already tried various tubes. As far as I know, Melos preamps operate in balanced mode internally, so it is not picking up RF internally. The RF might be leaking in to the input wires that run from the input rca's to the board itself. Any interference picked up here would of course be amplified. The RF might be local to you, which would explain perhaps why Melos did not find it or fix it. You might try ferrite rf blockers, either on the interconnects or the power cord, and if that doesn't help, then perhaps a torroidal choke on the internal wires (if you know what you are doing). OR, perhaps the internal wires are UN-shielded, like most preamps, in which case maybe replace them with ground shieled wires? Please try any of these at your own risk, keeping in mind that tube components can have lethal voltages in them, even when unplugged!
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