I've found the mains leakage to give 312V p-p (here in the UK), screenshot attached.
I then tried testing a whole bunch of 5V USB power supplies I had, and found they all (except one, but rated only 1.2A) leaked mains voltage pretty badly.
So I'll be making a little linear 5V supply for it.
I'm not getting much luck at getting a clean signal at low amplitudes (50mV).
Apart from that, a great bit of kit.
I received my JDS6600/25Mhz about a week ago and I am suitably impressed with its functionality. I have put it through some tests on a Siglent SDS 2204X 200Mhz DSO and it performs rather well. The two exceptions are a) the power pack leaks 78 V AC measured from mains earth to the DC ground of the power pack. Leakage current is 14µA. Not life threatening and can be fixed by replacing it with a transformer based PS.