I too, thought that the balances in his studio recording were a little odd (generally close up), but I'm able to overlook that and admire the performance, great as it is.
Remember, this particular recording was then done by a small, new English company that no longer exists (I forget the name without going to consult my Classic Record Quarterly magazines to find out, though it's definitely not Unicorn) that was taking bold first steps to record Mahler, so a little slack needs to be given them as we normally wouldn't be given Decca, EMI, or DG.
Wade