Agree with you, Bob, that the quad caps are not "deeply flawed." They have been successfully in use for decades and are still viable.
But, with regard to the Triode USA cap board for the Stereo 70, yes, sections two through four are rated for 500 working volts DC. But, the second section should actually see about 415 volts DC with the third and fourth sections less than that.
If someone’s wall voltage were so high that these sections rose to 500 VDC, then the amp might run into other problems as well (filaments, for instance), and then you’d have to find a way to get the wall voltage down (variac/thermistor/etc.), anyway. With my recent Dynakitparts PT (122VAC primary) and a thermistor, I ran my variac up to 130VAC and the voltage on those nodes were still well within the working voltage ratings.
Also, remember that each of the six electrolytics in the last three sections typically have their own surge rating that puts the surge capacity of those three sections at 600 VDC http://www.mouser.com/catalog/specsheets/panasonic__TS-UP_Rev2010.pdf
If someone were so inclined, a quick look at available values/sizes suggests that you may be able to select other electrolytics so that there would be a higher working voltage rating for sections two through four of the Triode USA cap board - if really necessary. This would likely mean reduced energy storage but perhaps not so much as to be of great concern.
I noticed that some of the Panasonic electrolytics are being discontinued. This datasheet shows what might be a replacement, but I have noticed a discrepancy in the physical dimensions for the caps used in the second section as listed on the Mouser webpage vs. the datasheet, which I have reported to Mouser. If a suitable replacement is not made, the Triode USA cap board might have to be changed in some way.
BTW, I have been in touch with both Triode USA and Sheldon Stokes, and the cap board Triode now sells for the Stereo 70 is not an SDS Labs design.