I don't believe you have the input impedance of your Rotel amp correct. Maybe 37K ohms. From the Stereophile measurements:
The RB-1090's input impedance measured 33.2k ohms (47.9k ohms balanced).Assuming you use the 10X rule of thumb, you'd want a preamp with a worst case output impedance (not the "nominal" value reported by the manufacturer) not greater than 3300 ohms (4700 ohms balanced). Stereophile always reports the worst case output impedance on preamps that JA measures, so their archives are a good resource.