First, a confession. Until yesterday I had never listened to The Beatles in mono. (Well, I own the Anthology discs, but I never listened to the mono tracks on there above once or twice.) I never even bought the first four albums on CD because I really like stereo; even early stereo recordings with their "analytic" division of tracks.
Most of yesterday I spent listening to the Mono boxed set on headphones, and right now I'm listening to a playlist of the songs for which I have 1987 stereo versions against their mono equivalents. (Always stereo first, which means that every time I hear the mono version I have to readjust to the acoustic shock of mono.) I believe that this is probably as unsympathetic an environment as one could find for comparing the two (short of repeating the experiment with 2009 stereo remasters). As most listeners will readily agree, mono works best if you listen to it without continually having to readjust from stereo to mono.
So far I'd have to conclude: the mono remasters are, taken as a whole, better than the 1987 stereo versions.
For about the first five seconds of every mono track, my reaction is always to think that the sound of the mono version is narrowed and muted, but that's purely a psychological response while the brain retunes. Generally, within seconds the ear is much happier with mono.
Some specific examples. The start of "All You Need Is Love" is horrible on the 1987 version: with the vocals hard-panned left and a nasty modulation whispering on the right-hand track until the strings enter, the sound is honestly amateur. Once your ear picks out that modulation sibilance it just leaps out for the entire length of the track. True, the strings are clearer and more detailed in stereo, but they are warmer in mono and the horrible tape noise just isn't there. At 24 seconds in on the stereo mix you can hear a studio voice in the right channel; you can't hear it at all in the mono remaster, and this may upset some listeners, but it pretty clearly wasn't "intended" to be heard. So: in the mono version you get the song as it was meant to be heard, with a choir in the background, and on the stereo version you get a small group of singers in the left channel and some horrible tape noise. It's not really a close call.
"Eleanor Rigby" is a clear case where mono should be preferable, because it has that hideous fade of the voice to the right channel on first word of the verse. The strings in stereo come from a point in space roughly centre: they are as mono as they could possibly be and really feel like it in the context of a stereo mix. By contrast, the strings work better in the mono remaster and have more bottom so that the cello sings a bit more. To be honest, I miss the separation here on the voices, but at least Paul stays where he is rather than teleporting into the right channel every now and again.
"A Day In The Life" is another track with pronounced modulation whisper in the right channel: it probably gets cleaned up on the 2009 stereo remaster, but the odd but deliberate panning of John's voice would remain as it is, and having Paul in the right channel for the middle section is peculiar on headphones. In defence of the stereo version, the orchestra sounds great even in the 1987 version. The mono version emphasizes the bass lines beneath the first verse, with Lennon's voice sounding recessed rather than panned. The piano in the first verse (such a feature of the stereo version because it is hard-panned left) is harder to follow in mono; surely indicative of a general blurring of instrumental detail when listening to the mono version. Overall, I'd say that the song was more pleasing in stereo.
Another song where the stereo has for me a slight edge is "Here, There and Everywhere", where the electric guitar chords on the second and fourth notes of the bar sounds unpleasantly emphasized on the mono version. I wonder whether this is a general feature of the 2009 remaster, because I can't see why this one instrumental texture should become suddenly brought into focus in this way. Perhaps someone with the 2009 stereo remaster could comment on that.
"Back in the U.S.S.R." has, for me, always been a good song ruined by the bloody airplane noises panning backwards and forwards all the time. Only in mono do these sounds properly bind to the mix, sounding (as I think that they should) like avant garde electronic musical effects instead of giving the impression that your listening room has been moved to a flightpath.
Obviously I could ramble on & on, but the main impression is that if you want to hear the music, the mono is the better option, and if you want to take apart the mixes (which are, let's be honest, rather fragile) you should probably go for stereo. The songs which really pushed available technology (such as "Strawberry Fields" or "I Am The Walrus") really sound awful if you listen to them carefully on stereo. It can never have been the intention for a tamborine line to have even more prominence than the vocal line, but time and time again in the stereo mix an instrumental line snaps into such focus in the soundfield that it's almost impossible to escape.
When my mono playlist started up on my bedroom system this morning it didn't for a moment sound as though anything was missing in terms of the music, so the loss of stereo separation is not something that is going to cause you as much distress as you might expect. Even on headphones (and this is with the crossfade circuit turned off for stereo listening) I can be listening to a mono track and not be at all conscious that it is in mono until I hear a stereo track for comparison.
