The criticisms I’ve heard of the Mercury sound in general are that it is ’dry’, it lacks a lot of warmth from the venue! That probably stems from the relatively close positioning of the microphones in relation to the conductor?
I have heard that a well-known British label is known to increase ambience by playing the original recording in a church, and adding reverberations back in.
Getting back to your recording, Gilbert Kaplan was a successful businessman, and got himself taught to conduct just to play this one symphony of Mahler’s. He was pretty darned good at it too, toured the world as an amateur and gave over 100 performances. He then learned the adagio from #5, doubling his repertoire.
I have CDs of his performance with the London Symphony Orchestra, which definitely does not use a small orchestra!
I remember hearing a stunning performance in the Melbourne Concert Hall. (Melbourne and Sydney have intense rivalries, including Concert Halls! Whereas Sydney’s has a world famous exterior but a bare-bones interior, Melbourne’s has no real exterior, being sunk into a huge hole by the Yarra River, but a sumptuous inside). There was a standing ovation, and the guy next to me turned and said he "wished his stereo sounded like that". I had just been thinking "this sounds exactly like my system".
Mahler 2 was the main work played after the $100-m refit of the Sydney Concert Hall, mainly done to fix the acoustics. Much of the refit is sculpted wood paneling to break up reflections! It worked ....

