Bob reynolds; the 40K assumes you subscribe to the 10:1 minimum ratio camp, and would probably be fine in many cases. However, after trying a whole bunch of preamps, I have found better luck when the ratio is higher. One can review the potential effect for their own specific equipment by looking at the dBr vs. frequency plot and interpolating for their own given amplifier load. The 3,900 ohm reading is at 20hz, so one might find a small drop in output at low frequencies is acceptable, especially if they are using monitor speakers, or speakers without much output below 35-40hz, since the drop at the speaker's low output limit frequency may be smaller than the drop at 20hz. Personally, if I were to use the Calypso with a SS amp, I would try to get something in the 80-100K ohm input impedance range, such as the JC1 monos which I believe have 100K ohm input inpedance. Ultimately, the main reason I sold my DNA500 was to be able to try a wider variety of tubed preamps without being handicapped by the 10K ohm input impedance.