The only thing "wrong" with this set is that it's a German release, and not easy to find in (what's left of) the American marketplace. The 24-bit remastering is not quite a night-and-day level revelation, but it does offer an improvement in fidelity over the previous incarnations of every recording here, and considerably so in the cases of the oldest of them, the symphonies 4 and 5, and ESPECIALLY No. 5 -- and that, in itself, makes the set particularly worthwhile, as Walter's 1947 account of the Mahler 5th has had a complicated, almost star-crossed history up to this time as a recording (even Sony Classical's efforts at an intermediate upgrade in the 1990s had to be re-done AFTER its initial release). As represented here, the Symphony No. 5 finally needs a minimum of apologies for its sound -- for the first time, there is a semblance of richness to go with the clarity of what used to be a thin, very dry sounding recording. The rest is glorious from start to finish, including two renditions of the Symphony No. 1 that are almost as different as night and day -- the savage, furious mono recording with the NY Philharmonic from 1954 versus the more lyrical and reflective stereo recording with the Columbia Symphony Orchestra from the early 1960s.
As to the matter of Walter's age at the time of the later recordings here, and the Philharmonic versus the Columbia Symphony, this is really a non-issue. Yes, Walter had retired in 1956, and suffered serious heart problems that slowed him down. But the offer of the chance to make these recordings, which embodied a series of sessions from 1958 until 1962 (the year of his death), gave him a new lease on his professional life, as well as a final mission, to address the arrival of the then-new medium of stereo sound and leave a legacy that would continue to be heard beyond his own time. He rose to the occasion, as did the players involved. And that brings us to the "Columbia Symphony Orchestra" as represented here -- it was comprised of many of the finest musicians on the West Coast, a good number of whom gave up higher-paying engagements at the time simply for the opportunity to make these recordings with Walter. And most of them, already at or near the top of their respective professions, outdid themselves -- don't forget that at this time, recording sessions devoted to Mahler's symphonies were rare and special occasions, and that classical recording was only just entering its boom time, when the advent of stereo precipitated the re-recording of virtually every piece of music under the sun that still mattered to audiences in the late 1950s/early 1960s -- at this time, EVERY session was special to some degree, and the quality and musicianship and the outlook of the generation of musicians then participating made recording on repertory like this even more important, as an opportunity that they all knew had eluded their predecessors (and who would ever have thought such opportunities, to record the Mahler 1st or the 9th, yet, would arise on the West Coast, and under this conductor [the man who introduced the 9th to the world, and visa versa]? It was a gift from God for many of those concerned). The tendency wasn't just to rise to the occasion, but to soar wherever possible.
Additionally, the rehearsals and the recording schedules were planned specifically so as not to tax the conductor's strength -- as a result, the notion that Walter was bringing less than his optimum energy from that point in his life to the podium is simply not relevant on these performances (and only became a consideration in his final round of recordings, somewhat later than those represented here -- principally one or two Haydn symphonies and the Bruckner Symphony No. 7, if memory serves). For its part, the New York Philharmonic was, of course, a powerhouse ensemble, and there was great and longstanding love between Walter and the players, but there is no reason to look askance at the Columbia Symphony recordings -- they were a true labor of love and a special opportunity for all concerned. Walter went into the latter knowing that these would be his lasting (and last) testament to the world about his way of interpreting the works involved, and given his relationship to the composer and the pieces -- and one must remember that he conducted the premieres of Das Lied Von Der Erde and the Symphony No. 9 in the second decade of the twentieth century -- they can all be regarded as being but a single step away from authoritative, where they are not absolutely the latter. The whole set is magnificent. And that Fifth Symphony is a wonder.
It's also a sign of the times we live in that these recordings, which were done primarily with the American listening market in mind -- until the mid-1960s, Columbia Records did not have an overseas operation, and licensed its recordings to Philips Records in Europe (which is how Columbia's Epic label in the US got the work of I Musici and the Concertgebouw Orchestra, in return) -- are being offered in this optimum remastered form to European listeners, and must be imported to the United States. Sony Music in America ought to be ashamed of itself for that decision.