How to go from RCA to XLR?

I'd suspect so, but I'm not sure that would necessarily be true with all equipment. Try it and see. If you get continuity to one of the xlr pins, you've verified that that pin is ground. If you don't get continuity to any pins, then that approach won't provide you with an answer. Same goes for measuring to chassis.

By "continuity" I mean something very close to 0 ohms (or at most say 1 or 2 ohms), as measured on an ohmmeter, not just an indication from a buzzer or other rough continuity checker which could indicate continuity even if a significant resistance is present.

Also, when you measure it would be prudent to have all signal cables from other pieces of equipment disconnected from the amp, so that your measurement is not confused by any possible paths to ground through the other equipment.

Regards,
-- Al

The standard for US equipment is pin 2 hot. Europe and most japanese brands use pin 3 as hot. It really doesn't do anything but reverse phase if it is not same, as Al stated.

Got some really bad news from Aragon tech support. Changing the XLR to XLR cable to an RCA to XLR cable, will cause the amp to run at only half power. Said half the input signal is grounded out so the amp, so the amp only puts out half power. Great.

Sounds like you need a single ended to balanced transformer like the Jensen model.

With designs that I am familiar with, feeding a balanced differential amplifier input with a single-ended signal will only mean that you have to turn the volume control on the preamp up a bit higher, by 6 db. It will not affect the maximum power output of the amp, only the volume control setting on the preamp that is required to reach that maximum output.

I suspect that the person you spoke to at Aragon doesn't know what he is talking about. Or else their design is very unusual in some respect.

A transformer such as Rwwear suggests will get you back most of the 6db, if that matters, but a quality transformer is likely to cost significant $, and even a good one may still have subtle sonic effects.

Regards,
-- Al