By a strange coincidence two M6s hit my doormat a few days ago - both are American, mid-60s and last with 2 secs of each other. They are Cleveland/Szell and Boston SO/Leinsdorf, the latter on a Japanese RCA CD. I think I'm right in saying it has never been on CD before. I have listened to both now.
The Szell is a famous recording which I had actually never heard before. This was the Great Performances incarnation, which I believe improves the sound somewhat over the Essential Classics CD. I didn't find this too enthralling, a solid, well played account which I suspect was one of the best around when it first came out but now is eclipsed by many more recent versions.
But the Leinsdorf was a bit of a revelation - I've never really been interested in his recordings (I have his second Madama Butterfly on SACD) but this is spectacular - whether the LPs were this good I can't say but this Japanese CD sounds magnificent, rich, vivid with excellent depth and a wide soundstage. The percussion has a presence which I can't recall hearing on many modern Mahler CDs or even SACDs. On the cover the BSO are hailed as 'The Aristocrat of Orchestras' and they sound superb here.
At the time of release this recording was quite well-received, much better than his M1, M3 and M5, but some felt that Leinsdorf's quick tempi missed some of the inner depth of the work, but I didn't feel that. Nor did I actually think the tempi were fast, at least compared to modern M6 recordings. The performance is quite extrovert if not excessively wild, but never threatens to become a 'Mahler-as-concerto-for-orchestra' as some feel, say, Solti sometimes did. I might say that Leinsdorf excels more at the faster music, so the Andante (placed third) has been done better by conductors like Karajan and others.
The detail of the recording make it very interesting and compelling for me, and may now sit near the top of my favourite M6s alongside Dohnanyi, Abbado II and MTT.