Balanced to RCA interconnect--is it beneficial?

no benefit

I tried this between my Spectral pre and Classe monoblocks and heard no advantage.

Dr. Richard Bews:

Balanced operation has advantages in environments of high noise or where cables have to be very long. Studios are a good example. The domestic environment normally doesn’t require balanced operation from a technical viewpoint. Balanced operation came about, because certain US manufacturers could give some technical bullshit to sell their amplifiers!

Balanced operation invariably adds complication, which can lead to problems, so why make the life of an amplifier more difficult than it needs to be?

It’s difficult to make a simple amplifier sound really good, so doubling the amount of circuitry (as often happens in balanced amplifiers) just makes it more difficult. When (our) amplifiers interface with amplifiers with balanced inputs or outputs, we prefer to use specially designed transformers to convert between single ended and balanced operation. This is simple, but does put demands on the circuitry and the design of the transformer.

All other things being equal anyway, Balanced to Balanced would be the most ideal. Single-ended to Single-ended would be the second most ideal. Balanced to Single-ended would very likely be the least ideal of the 3. In theory, this last situation is not likely to be any noticeably worse than Single-ended to Single-ended, since in effect that's really all it is, but in practice sometimes ground problems can come up in some systems as a result. For my money, Balanced to Single-ended wouldn't be recommendable if it costs you any money to try it. If you already own the needed cables you could satisfy your own curiosity, but spending anything to do so in my view wouldn't gain you any increase in performance...and just possibly it might result in hum problems as well.

No, as it is not a balanced circuit unless you are going Balanced to Balanced. The gear and how it was engineered determines your success rate with RCA or XLR connections.