I was doing some preamp testing and measured the output of my S-A-A-2 at about -10dBm. I didn't bother with peak-hold or anything like that, just hooked it up and looked at the screen. I think there was a bit less signal at the higher end, but only by a couple of dB.
The input of my unit does not like signals much higher than about -5dBm. It seems these devices really expect a passive (i.e. <0dB gain) device to be connected. I had to use one or two 10dB pads on the input of the preamp to reduce the levels so the NanoVNA would produce accurate indications.
It looks like a later version of the NanoVNA has a selectable output level, but even reducing it by 12dB doesn't help with a preamp that has 18dB gain.
Bob M.