This time I succeeded. But again, the switching PSU of the Perseus created problems. When connected to the same 4-way AC outlet as the signal generator, the PSU created noise that the signal generator picked up and distributed back to the Perseus so I actually needed signals of -80 to -90 dBm to get a 10dB SNR! Another reason to replace it with a stabilised PSU. When I placed them at different AC outlets, noise was gone. Funny that the Perseus itself appears to be immune to the noise.
But I got my measurements, didn't I? Indeed I did and they show a uniform -102 to -103 dBm sensitivity in AM, 6 kHz bw, 30% modulation with AGC off. Preselector, Preamp and Dither were all on. When I switched Preselector off, sensitivity increased a couple of dB, typically -103 to -105 dBm. While the Perseus (or the SDR-IQs) doesn't match my Icoms which have levels around -111 to -113 dBm, it should cope well even when DX-ing at very low signal levels once a high-quality preamplifier has been set in front of it.
I tried to remove the Loran C notch filter to see how the Perseus coped with it. Same result as with the IQs: Severe overloading on MW. But I hadn't expected otherwise.