Not sure I agree with extra reverb being added to that space/recording. If you look carefully you will see that room is larger than first thought. Why would you add reverb to a recording that already has nice reverb on it. Unnatural decay Hmm...  like tubby.. opinion, not fact. U87 Neumanns can do almost anything almost all of the time.
 
Now is not the time to get into a technophobe argument about the merits of ribbon mics and placements and pres because this recording has already achieved a big tick in terms of the performance seriously impressing the listener.  No amount of argument about how it should be done will change that one little bit.
 

Not sure I agree with extra reverb being added to that space/recording. If you look carefully you will see that room is larger than first thought. Why would you add reverb to a recording that already has nice reverb on it.

 
Not opinion, experience.  Backed by hundreds of live classical music recordings.  In a space that size, with mics placed that close, an instrument that size cannot produce enough acoustic energy to generate reverb of that volume.  Here is a little industry secret.  Just about every major classical release since the latter half of the 20th century has artificial reverb.  It is standard practice.  

Hey Jeff, I went back and listened through headphones and now agree that there was no reverb added (at least I think so).  However, I do think the overall recording has a tad too much ambience anyway.  I like a little more direct sound for this type of music.
 
Jimmy

Well Jimmy, Daniel could be very right too and I find that a bit strange. A lot of the time we are trying to keep the ambience down and get more detail from the instrument/s itself.  Are they doing this to make the spaces appear larger than what they originally were so to create the impression of a more important situation. Not sure.
 
I have certainly made classical recordings in rooms that did have that much natural ambience so that is why I thought it could have been real.
 
In retrospect I may have used the spaced pair plus another mic closer in to capture the detail slightly more so. (distances being measured too in case you have to compensate for timing differences) That is why I like M/S setups so much, you can control all this after the event.
 
Still overall a great recording though.

"That is why I like M/S setups so much, you can control all this after the event." 

Now that I listen to it again, on a better system, I'll take back tubby comment, it is fine.  I still would like some more high frequency sparkle.  I can understand how they were mistaken for ribbons.  87's are usually a gritty, edgy mic (in a nice way, I would never say it is a bad mic), but sometimes not appropriate for this type of recording.  Either the guitar was ridiculously mellow, or the top end was tapered in post.  
 
I'll blame it on Bose earbuds.  A heavy dull sound, but extremely comfortable, and don't fall out at the gym.  
 

Well Jimmy, Daniel could be very right too and I find that a bit strange. A lot of the time we are trying to keep the ambience down and get more detail from the instrument/s itself.  Are they doing this to make the spaces appear larger than what they originally were so to create the impression of a more important situation. Not sure.

 
Well I mentioned it is standard practice, not necessarily "good" practice.  Much of the time I am forced to add more and more reverb at the request of the client, and I hear the same story from many other recordists.  It is rarely the engineer's decision to wash out a nice and punchy recording.  
 
With Naxos the engineer also does not have as much control of the session as he/she would like.  In the old Decca/EMI days if the Tonmeister gave a command, it was obeyed without question, now we have unions, agents, large egos, and Zoom recorders which make everyone an "expert," reducing the poor recordist to the level of "sound guy."
 
 

This is what I hear.
There is both high and low pass. It could be the Mics, but I doubt it.
There is some reverb happening between the left and right sides that became quite noticeable when I listened in cans to one of the other pieces that had single notes played with a second or so silence either side of them, so I can only assume that it's still there in the rest of the recordings. The sound definitely phases in the stereo field.
The compression is delightful, I wonder if it was done going in? Probably.

There is both high and low pass. It could be the Mics, but I doubt it.

 
A high pass is usually a given considering the lowest fundamental of the instrument and the environment.  A few fellow engineers claim adding a low pass filter set around 18-20kHz helps with mp3 or online distribution sound. Maybe they consider it a preemptive, higher quality method, to the natural destructive nature of compressed audio formats.
 
Most good quality reverbs are pretty even in terms of stereo stability, so maybe the modulations on the decay could be a result of the room.  Just guessing there.    I have done plenty of recordings in wacky, ping-y, spaces.