A bit more info.
I did a few string swaps to try to track down what was going on; particularly with the E string on my NYC, which just sounded “tinny” with the RX Steels on my NYC. First I just put an older Hi Beams E on, which restored the NYC’s low end dignity in short order. Then, I put the misbehaved RX E string on my Fendowsky 4; and suddenly that string sounded quite normal. Finally, I put the RX E, which came off the Fendowsky 4 on the NYC. Which then sounded fantastic.
So, what happened?
I’m kind of guessing; but, my supposition is that the hex core strings are somewhat less tolerant of any twist during installation. The RX E string I had originally installed on the NYC may have been ever so slightly twisted. No real way to know; but, that’s my guess. I’ll keep forging ahead with the hex cores, which sound fantastic and seem to have much longer life. If I can correctly install them.