Always mighty, often amazing, the Mahler

Symphony No. 8

unfolded Thursday with somewhat less than the supposed thousand musicians for whom the piece was ideally conceived. But you wouldn't have wanted more than the Philadelphia Orchestra's 420 singers and instrumentalists, who made as much sound as the Kimmel Center's Verizon Hall could hold.

The first of four sold-out performances that promised to be (and were) the season's highlight, the event commemorated the 100th anniversary of the orchestra's U.S. premiere of the Mahler 8th under Leopold Stokowski. But the piece needs no occasion markers, just money for the extra voices that filled the hall's Conductor's Circle section with choral forces, as well as some of the best vocal soloists in the business.

Certainly, the experience was sublime, transcendent, all the ultra-superlative things one saves for a piece like this. Music director Yannick Nézet-Séguin makes choral music his specialty, and the orchestra, Westminster Symphonic Choir, and American Boychoir were all in top form. But here's the paradox of the Mahler 8th: Both the performance and symphony itself can feel questionable as you're taking it all in yet remain utterly compelling.

As might be expected on Thursday's maiden performance, it felt like a dress rehearsal. The symphony poses singular challenges. The symphony feels like an assignment from outside - God, perhaps? - as opposed to something that welled up from Mahler's personal needs. Herein lies the challenge for the performers: Beyond the symphony's broad philosophical strokes about the connection between spirituality and creativity, what is everybody singing about? In Part One, Nézet-Séguin didn't fall back on all-purpose majesty. In moments with few singers and an extremely quiet organ, the relative silence had more magnitude than any of the fortissimos that followed.

Quiet intensity was also in Part Two - a vision of heaven lifted from Goethe's Faust, where the meaning can seem poetically vague. Some soloists seemed score-bound to various degrees, namely Angela Meade, Mihoka Fujimura, Marcus Werba, and John Relyea, although in all fairness, those last two were placed at an acoustic disadvantage. Anthony Dean Griffey made efforts to express the words, but his lack of gravity suggested that, unlike his German counterparts, Goethe isn't in his bones.

Significant exceptions were Erin Wall and especially Stephanie Blythe, who marvelously commanded music and text. A special word for Lisette Oropesa, whose character, Mater Gloriosa, is supposed to appear out of the sky. The top tier of Verizon Hall was fine, with her in a white gown and sounding gloriosa.

Additional performances: Sold out. Information: 215-893-1999 or philorch.org.