That's a great question. When 2011 rolls around, I think you'll see some very complete Mahler editions.
"I've owned the Haitink/Bruckner boxset and was displeased at its lack of bass, making the whole cycle 'light'. I have downloads of M5, and M9 from the Haitink/Mahler boxset and also notice a lack of bass. These cycles, well received as they are, would have been even more so had they had 'muscle on their bones'. What a tragedy!"
I couldn't agree more. However, Philips just reissued Haitink's 1966 M3 with a really good remastering job. There's a sufficient amount of bass too. It's coupled to his fairly decent "Das Klagende Lied". His first M7 - one of the better recordings from Haitink's original cycle - has never been issued separately. I'd love to see Philips give that a good make-over as well.