As you suggest, there are pieces where the S isn't too high, e.g. If Ye Love Me by Tallis.
Conversely, you can look for pieces written for TTBB or ATBarB have them work for AATB, sometimes by singing them a little higher, e.g. Gratias Agimus Tibi by Hassler, or the lovely Ave Maria by Cornelius Verdonck.
There's also some nice pieces for ATB, e.g. Panis Angelicus by Casciolini.
A number of the motets of Orlando di Lasso will also work. I have before me his SATB "Domine in auxilium." In the original pitch version the highest note in the Cantus is d.
Depending on the range of your members, you could possibly lower SATB arrangements into an AATB range. Just bring it down a third or so. May be a darker sound, but heh! The Russians make a great choral sound.
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