Astral Tether states: "You can do so only if the amp grants a distinct benefit, not if it alters the amped spell."
Ghostly Shift Amped effect is a distinct benefit that alters the amped spell...
Do you think they should interact together or not?
In my opinion, the benefit is distinct enough so it should work. But I can see a case made that it's an alteration of the amped spell (which all amps are unless you grab them through a feat so it makes Astral Tether half pointless).
I think the only amps that grant distinct benefits rather than altering the amped spells are the ones granted by feats. The feat amps are all distinct benefits unlinked to the spell. The core amps of a cantrip all inherently alter the way the spell itself functions.
So the intent is that you can apply the amp that gives resistance, or the one that makes a target unable to see you. Make your bard invisible to the bbeg while he harmonizes some buffing compositions in a stationary position, or inspires courage plus throws the occasional Heroism or Soothe.
Yeah, that one is weird.
The Amp benefit seems to be distinct from the spell, but is also dependent on the spell's base effect. So since the Amp effect is dependent on the spell's base effect, then it isn't really distinct.
Astral Tether is really badly written. If the intent was to use it only with the feats amps, then it would have been easy to state it that way.