There are a couple of Mahlerfest performances with 'historical' value. The complete M10 is the first recording of the Joe Wheeler performing version (as edited by Olson et. al.). Olson went on to record it again on Naxos (Polish National Symphony?).
At the recent Mahlerfest last month (8th Symphony), I picked up the just released Mahlerfest recording of the 1880 'original' version of Das klagende Lied (from two performances in Jan. 2008, both of which I attended). To my knowledge, this is only the second recording on CD of the 1880 three-part original after the Kent Nagano world premiere (there is also a Vladimir Jurowski version out on DVD now).
I really enjoyed the recent three performances of M8 on May 15-17 in Boulder (the first being the dress rehearsal). My only quibble, similar to 2008's DkL, is that the choral forces, particularly the male voices, were a bit overwhelmed at times due to sheer lack of numbers. This becomes apparent at several spots in part 3 of DkL--I haven't listened to parts 1 and 2 yet, but this confirms the impression I had at the live performances. But it's hard to complain when you consider the largely volunteer nature of the Mahlerfests over the past 22 years, and the limited stage space of Macky Auditorium, which did however boast an excellent organ for the M8 concerts.
By the way, Olson's very deliberate pacing in DkL offers an interesting contrast to the much quicker tempi of Jurowski.
Mike Bosworth
Hanoi
