Mike Leghorn replies to me:

>>Except the sequence is not in the Grosse Fugue.  There *is* a four-note
>>motive, but it's not c-d-f-e; rather (if the quartet were in the key
>>of C instead of Bb) G#-A-G'-F#' in its opening form.  Furthermore, it
>>never becomes c-d-f-e in this piece.  So now, in effect, we've come from
>>"Mozart's 4-note theme," to any 4-note theme that rises by a whole or
>>half-step, skips x number of notes, and falls a whole or half-step.  It's
>>a real stretch to say that anyone after Mozart who constructs this kind of
>>motive gets the idea specifically from Mozart's Jupiter (since the motive
>>occurs in other Mozart works) or even specifically from Mozart.
>
>Are you saying that there is no relationship between the sequence at the
>beginning of op. 132 and the 8-note theme that is the basis of the Grosse
>Fugue?

Not at all.  In fact, ever since you began this thread, I've been
listening to the quartets with the general shape of the motive in mind
(thanks for that, incidentally!).  I do hear the shape throughout these
quartets.  My only question is whether Beethoven is in aesthetic dialogue
with Mozart.  If so, is Shostakovich when he uses D-S-C-H (especially in
the form D-Eb-C'-B)? It's a coincidence, but is it really a coincidence
worth pointing out? That is, do these four notes point to Mozart? It seems
to me that the function of these notes is quite different, more on the
order of B-A-C-H (Bb-A-C'-B) -- in other words, it's musical iconography,
signature, but probably not the finale of the Jupiter.

>In fact, Deryck Cooke, in his article "The Unity of Beethoven's late
>Quartets" proposes that the last five quartets "constitute a single
>continuous act of creation".  In this article, he sites Nottebohm's
>assertion that "Beethoven had used the same pitch-pattern as a principle
>thematic idea, in the first movement of no. 2 in A minor [meaning op.
>132] (as a kind of opening motto-theme) and in the finale of no. 3 in B
>flat [i.e.  the Grosse Fugue](as a kind of opening motto- theme, and as a
>subject of the Grosse Fugue)".

All respect to Deryk Cooke, that opening to op. 132 is a little vague
(essentially it's more harmony than motive), but I'd love to know how he
argues for it.

>To my ears, the opening to op. 132 is closer to the Mozart trio than it
>is to many of the other occurrences the 8-note theme throughout the late
>Quartets.  What does this prove? Nothing.  It's just a theory.  Theories
>can't be proved.  They can only be disproved.

Actually not.  Theories can (or not) take into account various facts.
The more they can take in with a minimum of fuss, the more attractive
they become.  The less they take in or the less elegant they are, the
less attractive.  They can also lead (or not) to further questions.
Nothing dooms a theory like a "final answer."

Steve Schwartz