How to go from RCA to XLR?

Rww -- yes, but they still could have provided balanced xlr inputs for each stereo channel, and then fed those inputs into a pair of differential receivers with single-ended outputs. The single-ended outputs could then have been fed into the same path that the single-ended rca inputs in the actual design are fed into. That would add the benefits of a balanced interface and cabling, without changing the internal architecture significantly (other than the addition of a few components).

they are using the two separate channels combined to make the amp balanced .... That's the reason you probably should not use an RCA adapter to convert an amp to single ended in this type of design.

Well, ok, I don't have sufficient familiarity with this type of design to comment on this.

Regards,
-- Al

See pages 28 and 29 of the following paper for a description of how to feed a single-ended signal into a balanced input without major degradion of noise performance. My thanks to Shadorne for calling this paper to my attention.

http://www.jensen-transformers.com/an/generic%20seminar.pdf

Regards,
-- Al

My thanks to Shadorne for calling this paper to my
attention.

You're welcome - Bill Whitlock used to work at Capitol Records - he is a member
of IEEE and AES. What he says is obviously designed to help raise awareness and
promote Jensen products but he shoots straight! I would not hesitate to use his
products.

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

Rww -- I get your drift; touche! :)

But the op should note in the reference that he can get a great deal of noise rejection just with the cabling arrangement that Whitlock describes (his diagram labelled "correct" on page 29), without the expense and possible sonic signature of a transformer.

And as I indicated earlier, even if he just uses the adapter as originally envisioned (Whitlock's diagram labelled "wrong" on page 29), he would have noise performance comparable to a fully single-ended arrangement, which may prove to be satisfactory anyway.

Regards,
-- Al