I haven't powered it up yet, I do not have a dim bulb tester.
No need for a dim bulb tester as long as you simply replace all the caps and any out of spec resistors first. If you have, in fact, rebuilt a Magnavox then this amp will not be anything you can't handle.
I'm not familiar with this particular amp but it looks like a very basic circuit with very few parts. Yes, the electrolytic power supply caps and all the other gray caps in these Japanese amps are notorious for being out of spec. All of those caps need to be replaced before you go any further. Measure the resistors to see if they're in spec. Or, just order all new ones since there aren't that many.
Here's a link to a thread I posted a while back which explains how to get free shipping from Digi-Key.
http://www.audiokarma.org/forums/in...hipping-on-parts-orders.739513/#post-10018562 Shipping charges can raise the cost of small parts orders significantly so, if you're not in a big hurry, you can save a bit of money.
Yes, the 6X4 rectifiers are prone to failure when they are run too hard. Adjusting the circuit so that the current draw doesn't overstress the rectifier tube is not that difficult. I'm sure the output tubes use cathode bias which consists of a resistor and (possibly) a bypass cap on the cathodes of the output tubes. It's easy enough to solder in a different (higher) value resistor. If the current draw is too high in stock form this will reduce it. No more difficult than changing any other cap or resistor. If you're ordering new resistors just order a few values that are a bit higher than the stock cathode resistors so that you have options.
I would suggest that you replace all the caps first, put together a dim bulb tester (simple and cheap) and use it to power the amp up in otherwise stock form. Then you can use your meter to measure the actual current draw, instead of speculating on how high it might be. If the current draw is too high, simply solder in some different cathode resistors until it is no longer stressing the rectifier tube.
You could also replace the rectifier tube with some diodes (UF4007 should be fine) which are very cheap. Adding a part called a thermistor (CL80 or CL90) to limit the inrush current on startup would be a good idea if you go that route. You may also need to change a resistor or two in the PS to adjust the voltages too. I'm sure there are more sophisticated circuits that will provide a slower startup but thermistors work pretty well. Keep in mind that most vintage amps that were designed with SS power supplies didn't use anything to limit inrush current and voltage.
Yes, there are some issues surrounding the use of the 6X4 but the sky is not falling. The amp is, what, 60 years old. If it hasn't blown up by now it probably won't blow up immediately the next time it's powered up and by making a few parts adjustments you can avoid that scenario entirely.
Perhaps someone has a schematic and will post it. That would be helpful.