While, sadly, I am not familiar with the Marx quartets, I can answer
Mitch's questions about modes.

A mode is a scale and a tonality.  You may have heard the term "diatonic",
which refers to the tonality which arose during the Baroque era, based
only on the major and minor scales (The Ionian and Aeolian modes).  The
Mixolydian mode is like a major scale with a flatted seventh, or a scale
using only white keys on the piano, and beginning and ending on G:
G-A-B-C-D-E-F-G.

The Phrygian mode is like a natural minor key scale, but resting on
the fifth note of that scale.  It may have a sharped third (hence the
"Phrygian" cadences which end many Baroque-era minor key slow movements).
So if we try to use white keys again, E-F-G(#)-A-B-C-D-E.  A lot of
folk music uses Phrygian with a sharped third, or related modes, e.g.
Scandinavian, Hungarian, Spanish, and so they frequently pop up in the
music of composers like Albeniz, Bartok, and Grieg.

Michael Cooper