Hey Wes keep it going! and keep reading!! The voltages on caps is their maximum voltage, and their lifespan is greatly reduced if you exceed their rated voltage, a 350v cap may last ~80% of its rated 3000h lifespan at 400v, or burn out in a couple hours at 500v. In general you can't go wrong with higher voltage ratings, as long as they fit, and caps of the same rating generally get smaller and smaller as the technology improves. Because of this my old PV amp now has caps of identical size to when it was made but 2-3 times higher value in micro farads (uF). More filtering makes the amp quieter and "stiffer" sounding and generally has no down side unless you have a tube rectifier (which few modern amps have)
The separate power supply board is where the 220uF 350v snap in caps go, but notice the caps are connected in series: the C45 negative terminal (dark box) is connected to the C46 positive terminal (open box)
+(C45)- +(C46)- Ground
parallel caps have a total capacitance equal to 1/220 +1/220 = 1/x so the total capacitance C45 +C46= x = 220uf but the trick is that the voltages are now additive, so you've turned 2 x 220uF/350v caps into one 220uF/700v cap!
this is essential cause you can't buy a single cap which will work, the R71 and R72 resistors are sometimes called "equalizing resistors" and serve to make sure each series cap sees the same voltage, otherwise one could end up with 90% of the voltage and burn out way fast, they also allow the charged cap to discharge safely when you unplug the amp; a capacitor without a resistor across it can store a charge for a long time and discharge instantly when touched and kill you. Bigger value power supply caps are the dangerous ones; a 0.022uf cap holds very little charge!
The way a tube amp power supply is (almost always) designed is to take a VERY high B+ voltage and feed it to the power amp tubes plate, a bit lower voltage to the power amp tube screens and then much lower voltages to the preamp tubes. All on a single rail from the power transformer (PT) through a series of series resistors which drop the voltages. B+ plates/screens/preamp tubes. The very low heater /bias and accessory voltages usually come off different PT taps and separate rails.
You should feel comfortable with series and parallel resistors and caps. Voltage ratings on caps are minimum values, and cap values are MUCH more important in the signal path away from the power supply.
Power supply cap 220uf vs 330uf little if any difference
Signal path coupling capacitor 0.022uF vs 0.047uF big difference!