if the high voltage finds a path to ground. Your plan is to DC float the circuit, but not AC float it. Right? The cap? All I can do is compare to 2 prong DC floated chassis on the old amps which I am very familiar with. Somehow the, the DC on the chassis stays within a few volts of earth ground level, unless of course the high voltage shorts to the chassis which is a rare occurrence. This only happens when the 2 prong Plug is oriented wrong and the chassis is hot with wall voltage. I wouldn’t worry about the voltage exceeding the B+, personally. 400 volts is bad enough. Higher voltages can even be safer. It burns the flesh so the electrical connection is lessened. But again, if you are not grounded, the voltage could be 500 or a 1,000 and you won’t feel it at all. That reminds me of an interesting experiment that I have either done or nearly done. Make sure you are wearing shoes. Open up the amp circuit and turn the amp on. You can do this standing on concrete, but if you want to be really safe, stand on a non conducting surface. Then, with one finger, touch the plate of the power tube. Nothing will happen. Variation: with one finger or one hand, touch the power tube plate and the chassis at the same time. This is also safe because the electricity travels thru your hand which is no big deal.