As you point out, there is no need for 630V, they are just that rating because more and more low-voltage series have been taking out of production (or not kept on stock) For the snubber caps around the rectifier diodes, assume that they could face full Vpp of the transformers' AC, so 200V would be the minimum to take Capacitor voltage ratings come in standard values like 250V/400V/630V Although multiple voltage rankings exist on the datasheets..... retailers don't like to keep each uF/V item in stock, so ..... you simply end up with what is available, not what you theoretically would like to get. Same issue with the Nichicon FG series: 50V and 100V are normally available at Mouser, but the 63V version is often out of stock. Yes, finding the parts with the right dimensions (leg space & overall) versus available capacitance and voltage rating can be 3x more work than actually putting them in. As for rectifier snubber caps, keeping inductance (ESL) low is also key, that's why short legs and installing them as near as possible the rectifier legs is the best practice. Which is also the reason why ceramic disc capacitors are often used for snubber functions: they have a much lower ESL than film capacitors. You can see in many Yamaha amps (CA-1000/2000, CR-2020, etc) that they used ceramic disc caps for snubber functions So if you can't find the right snubber caps in film format, don't hesitate to go for ceramic disc caps. As for the bypass caps around the transformer secondaries (the two existing green mylar caps near the rectifier diodes) , try to stay with a polypropylene film cap.