Agreed, George. I was also envisioning an approach using a line-level switchbox, as described below.
Zavato, before I get into that I want to mention a point that I think should be kept in mind. For some and perhaps all of your source components the RCA and XLR outputs are probably not independent of each other. It is not uncommon for components providing both kinds of outputs to provide the same signal to the RCA output connector and to the non-inverted signal pin on the XLR connector (usually XLR pin 2 in the USA). The manual for your Pass Aleph Ono, for one, has wording indicating its design is done in that manner.
Although as a practical matter it may not make a perceivable difference, it is therefore most ideal to not have both outputs connected at once. Otherwise there MIGHT be at least slight effects on sonics as a result of introduction of unequal amounts of noise on the two balanced signal lines (which would therefore not be rejected by the balanced receiver circuit), and as a result of having unequal impedances on the two lines (which degrades common mode noise rejection), and as a result of having unequal loading of the signals on the two lines. The approach described below pretty much eliminates those possibilities when listening via speakers.
My suggestion, assuming that the INT-150's mute function disables its pre-outs and therefore makes them unsuitable for connection to a headphone amp, is as follows:
1)If you already haven't, determine whether your two most critical sources (presumably phono and CDP) sound best when using their XLR or RCA outputs. The following assumes that their XLR outputs sound as good or better than their RCA outputs. If that is not the case the suggested connections can be revised accordingly.
2)Buy an RCA line-level switchbox that can accept at least four inputs (one more than the number of sources). The Decware ($279) suggested by George looks like an excellent choice. The DB Systems
(follow the "Accessories" link) model DBP-2JAU/5 ($110, or more with various options), which I used many years ago with excellent results, is also worth considering. Further info on it is provided here
3)Buy a headphone amp, preferably having an input impedance of around 47K or higher.
4)Connect the XLR outputs of the phono stage and CDP to the INT-150. Connect their RCA outputs to the switchbox.
5)Connect the RCA output of the tuner, for each channel, to both the INT-150 and the switchbox, via a splitter or y-adapter.
6)Connect the two output channels of the switchbox to the headphone amp.
7)When listening to the speakers, set the switchbox to a position for which no input is connected. The addition of the switchbox, the splitters, and the headphone amp will then have essentially zero effect on the signal path to the speakers!