Just one thing to maybe watch. Be careful if you have the scope on AC coupled input setting. I'm not sure how the inputs on the Siglent are implemented, but there is the possibility of input overload. If you quickly switch from probing a +400V node to a -300V one, you possibly have an [Edit: transient] 70V swing from 0V on the scope active input circuitry (after the x10 probe) due to the existing +40V charge on the input coupling cap. I haven't completely worked it through, but there is the potential for an issue there.
EDIT: Sorry, looking at your OP again and checking the SDS1104X-E spec. I see that it does have a 400V (DC+AC Pk), not 40V as I assumed, so not a problem... Err, it might still be if the 1M input resistor is after the input cap as is traditional on analogue scopes, you may have a transient 700V swing.