the noise comes from the output or headphone amplifier modules you use after the ssp. they have inadequate filtering so they pick up anything on the power rails, ssp or not.
you can deal with this by using a low impedance bus board like a LIBB, and low impedance power cables, and ideally a supply that gives you directly +/-12v from the mains, instead of just 15v which is then converted to +/-12v by a switcher on a bus board.
a lot of bus boards are not very well designed. you can find a lot of info about this online. both the guys from genus modus and hinton have written extensively about these issues.
i personally tend to use external mixers and amplifiers instead of relying on eurorack mixers. they are cheaper and often designed much better. mackie and others have designed mixers for decades, and have been around before eurorack even existed. just my 5 cents.
i have had good experience with roland SYR84 cases. putting the ssp or your amplifier modules on seperate power supplies is also a possible solution. that roland case has 2 bus connectors internally which makes it great. put the ssp on one connector and your other modules on the other connector.