I totally agree with you, John. For me, this one came out of left field and caused me to revisit Jansons' other BRSO recordings and sparked a renewed interest in his Mahler for me.
If you take all of his BRSO recordings, sub in one of his M6s, his M4, and his M8, you have one hell of a "cycle" already. Let's hope BR-Klassik gets back to me with info about them eventually putting an entire BRSO cycle together.
And about the tam-tams, I think the BRSO must just have a huge, fat, dark tam-tam. All the major tam-tam points in this M3 seem a bit "underwhelming," but if you listen closely you hear a very bassy, resonant, dark sound with super deep undertones. And this goes for the rest of the BRSO Mahler recordings, even the big tam moment in the first movement of the M9. It's just less "splashy" and more dark and imposing than other recordings we're used to. Just my own observations; I have no idea what tam-tam they actually use. And if you listen to Jansons' RCO recordings you can hear a lot more loud and splashy tam-tam hits, so the guy isn't just glossing over it. Same with his LSO M6 and his early Oslo recordings. In other words, I blame the general audibility on the BRSO's tam and not Jansons himself.