OP,
this is why a lot of McIntosh and other brands had 'trim' controls, to equalize the signal strength of several inputs, so the volume does not change input to input. Heresy, another attenuator in-line, out damn spot. It's a nice feature when needed.
have you checked the manual, I have had 2 players that had default 'fixed' output, and deep in the menu, 'variable' volume. My Onkyo had two sets of outputs for this, the Oppo a menu change.
btw, don't count that Sony out. My garage sale Sony Carousel (5 discs) just equaled a supposedly superior player's sound quality, AGAIN. Years ago, it surprisingly sounded as good as my big Onkyo dual matched burr brown beast; and now matched another supposedly superior player. I kept the convenience, slightly smaller size, placement options, smaller remote, easier to read display and sold the other.