Glare on Choral recordings, classical vocals

Glare is unlikely to be due to room modes (which would affect bass) but could easily be due to moving a dipole speaker so the tweeter back wave fires into a glass window (don’t ask me how I know that).  It is certainly not due to speaker wire insulation.

What changed to before/after the glare? Or did nothing change except you bought some new recordings?  It sounds like it is limited to a single label, but occurs on several recordings(?)  If so, that suggests the way their engineers are making a "house sound."

No matter what, I’d spend a few $$ on diffusors long before spending a lot on new equipment, tubed or not.

Well, I sing in a couple of choruses and have done so all my life.  In certain acoustics, the treble voices in our choirs can sometimes be overpowering and create the glare you're talking about in real life, even though the voices are excellent. I would not underestimate the effect that the recording acoustic would have on the sound you're describing.  Do you have any recordings on the Telarc label of the Atlanta Symphony chorus under Robert Shaw?  Do you hear that glare with that chorus?  I generally don't get much glare from those recordings on my system, but I sometimes hear what you're describing on recordings of English choirs (the Cambridge Singers, for example) in very reverberant acoustics.

I enjoy classical choral very much on my system. For whatever reason, the analogue material on Argo vinyl is beautiful, engaging, and non-fatiguing, while I never listen to digital vinyl for this material.

I can sometimes enjoy a Harmonia Mundi SACD by Anonymous 4, through a 10 KHz 6 db low pass filter.

Some might take this data as evidence against the 'defect in an individual system' hypothesis.

More thoughts on causes of glare: Could be due to an imperfect crossover configuration. I remember reading a review of a speaker (can't remember which one) where the author stated the crossover points were at 200 hz and 2000 hz. He stated these were close to ideal as they did not affect the vocal range. An improperly designed crossover (crossover point within the vocal range) can have different drivers producing the same frequency causing IM distortion. Check your speaker's specs.

Another consideration is the composition itself. Composers like to "reinforce" the vocals with instruments of similar timbre (e.g. violins).
This could give the impression of glare when it might just be poor composition or the inability of the speaker to precisely delineate the two sources.

"I've noticed that audiophiles and reviewers seldom write about choral music. Is it because they don't like choral music, or is it because it is just really difficult to record choirs well, therefore making choral music a poor choice to show off a system?"

I've noticed that too Paul and I personally believe that it is often for the reason you note above and others in their responses. I too love choral music and have focused my attention, along with large scale orchestral music around my system in making this type of music as natural and realistic as possible.Agree with Rcprince's observation about vocals, particularly lyric soprano voice can often come through with glare and IMO is about the hardest instrument to sound completely natural through an audio system. I too notice that vinyl recordings seem a bit more forgiving than do digital. I think your remedy could be some or many of the excellent recommendations given above along with seeking out quality recordings. I find close miked recordings of vocals seem more problematic.  Getting it right with this type of music has been in my experience "a long road" but your 20 year old impressions concerning a Conrad Johnson preamp certainly is a good choice for this type of music IME. 

Agree with Rcprince on the Telarc/Shaw recordings, less notable glare and more of a mid-hall presentation which is ok by me if the recording is rendered more listenable. It seems the biggest problem is the transducers themselves, microphones and speakers at least that is my suspicion.  On a final note most of the audiophiles I know don't particularly care for choral music with a few exceptions. I would think that anyone that would commit to purchasing a very expensive pair of loudspeakers would be really committed to attempt recreating large scale music and voice as realistically as possible or what's the point, listening to Rebecca Pidgeon singing "Spanish Harlem" as realistically as possible?