How to go from RCA to XLR?

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

Noslop, the news you got is erroneous! If one of the phases is not present, the gain of the amp may be reduced by 6 db, but there will be no loss in power! Whoever told you that really has no idea how this stuff works.