You could put all classical music in a separate folder that isn’t inside the folder Sonos looks at for your music library.

If you submit a diagnostic and contact Sonos Support (not here) you could have them give you your library storage statistics.

Beats guessing or doing a bunch of work if there is no problem.

 This is not a Classical tag, but all my tags are exactly the same.  I have over 1500 Classical composers and they all index just fine.  Keep your tags simple.

 Only compilations would have the Album Artist tag as “Various Artists”.

This got tagged as answered possibly mistakenly by me but I should be able to correct that but it does not seem I can.
 

 I don’t know how that’s done either.

I appreciate your answer and it could be part of my problem, but with an iTunes library of 35K songs I’m not about to start modifying all the files.

 Yeah.  I understand.  Did a friends tags on his Melco N1 which took about a year.  Did a little at a time.  Maybe a couple hours a week.  The benefit was having that awesome Melco N1 for over a year.  Great piece of gear.  

 In my friends library there were a lot of tracks/albums with a ridiculous number of tags categories filled out that serve absolutely no purpose.  Had to remove them all.  Not sure if your tags suffer from that same condition.

 I would never suggest you spend a lot of time tagging files.  In your spare time just knock off a few.  Proper tagging of files does make searching easier.  Of course yours may already be good.  What I would do is look at the tags of one of the Classical albums just to see if there are more tag categories than are necessary.  In my picture in my earlier post the only category I regret not using is Composer.  Old control Point software did not have a Composer search category.