Tantalizing ambiguity was the order of this past Saturday evening at Tanglewood’s Koussevitzky Music Shed, where a stylistically eclectic grouping of works by Ives, Strauss and Tchaikovsky all played with notions of death and eternity. Replacing an ailing Christoph von Dohnányi, Ken-David Masur led the BSO with a steady hand, projecting assurance in a program of music that delivers anything but.

I can’t help but remark on the uncanny timeliness of the opener, Charles Ives’s The Unanswered Question. Played against a backdrop illuminated in blue, white and red (presumably to honor the Bastille Day attack victims in Nice), the work’s understated, subtly eerie qualities seemed all-too-well-suited to this age of fearful uncertainty. Ives’s music comes across like an anxious attempt at meditation, the regular breathing of the strings disrupted by troubling, unbidden interjections from woodwinds and brass. The accomplished performance set a pensive tone that persisted to a greater or lesser extent throughout the evening.

Adding a bit of flash to these sober proceedings, guest star Renée Fleming sauntered majestically onto the stage to join the orchestra for Vier Letzte Lieder, Strauss’ haunting, valedictory song cycle. Clad in a glittering champagne-colored gown and a mauve/taupe floor length brocade jacket, the soprano cut a regal figure; though she sang nothing from Rosenkavlier, she looked every inch the Marschallin.

The first two songs of the set, “Frühling” and “September”, came off as subdued, almost to the point of listlessness. This was perhaps appropriate for the second piece, speaking as it does of unbearable weariness, but less so for the opening song depicting rapturous dreams of spring. Despite a general balance of forces between voice and orchestra, Fleming’s low range remained somewhat hard to hear at the start; one hoped that the Shed’s amplification system served audience members seated at the back of the house and on the lawn.

With the third song, the almost unbearably tender “Beim Schlafengehen,” Fleming reminded opera fanatics of her status as a leading Strauss interpreter. The full, rosy glow of her warm soprano lent sensuality to this monumental lullaby, exquisitely matched by the aching loveliness of Malcolm Lowe’s violin. With “Im Abendrot”, Masur guided Lowe, Fleming and the ensemble through to a serene yet spooky twilight decrescendo, the last note lingering on the air like a wisp of smoke.

Fleming readily obliged a wildly appreciative audience with an encore, Strauss’s “Cäcilie,” Op. 27, No. 2, permitting her to launch into the high drama intentionally absent from the preceding song cycle. Complemented by Jessica Zhou’s harp virtuosity, Fleming received an ecstatic reception from the Tanglewood crowd.

Despite a heartbreaking desire to master the symphonic genre, Pyotr Tchaikovsky never quite reached the same degree of accomplishment that characterizes his output as a composer of ballet and other stage music. This frustration is borne out by the style and substance of his Symphony No. 6 in B minor, Op. 74, “Pathétique,” which had its premiere nine days before the composer’s mysterious death in 1893.

Perhaps as a means of addressing structural flaws, Masur approached the evening’s final offering with Germanic gravitas, choosing to emphasize the verticality of harmonic blocks instead of the linear motion of individual motives. Though the tactic lent the symphony weight and moment it otherwise lacks, it may also have detracted from what Tchaikovsky provides best: melodies and melodrama. Still, the conductor imbued this reading with his signature attributes, an affecting combination of emotion with restraint, polish with fire, conjuring an array of timbral effects. Masur skillfully moderated an eloquent dialogue between woodwinds and strings in the first movement, eliciting a truly mournful solo from William R. Hudgin’s clarinet. Throughout, timpanist Timothy Genis gave the best among several magnificent readings from the percussion section, bearing full witness to Tchaikovsky’s often-thrilling flair for theatrics.

In an evening of solid musicianship, it is worth noting that the least impressive performance originated not onstage, but in the house. I am sorry to report that a significant number of Saturday evening’s crowd bore witness not only to our 21^st century culture of distraction but also to declining standards of concert etiquette and literacy. Though codes of conduct are understandably (and gratifyingly) relaxed at an outdoor summer concert, I was still taken aback by the sight of audience members Facebooking as the orchestra played, hissing commentary at each other in what might charitably be deemed stage whispers.

A second series of infractions involving ignorance of convention was perhaps less egregious, yet even more disruptive. Not only did the audience break up the flow of the Strauss cycle by loudly applauding between movements, but at the crashing final cadence of the “Pathétique’s” third movement, a significant number of the crowd leapt to their feet with the now-obligatory standing ovation. Since they failed to notice that Masur did not turn to acknowledge the house, one supposes the gesture was less a reflection on an admittedly fine performance than a desire to telegraph enjoyment to the rest of the room. Astonishingly, a number of attendees rose from their seats and made their way towards the parking lot even as the ensemble began the fourth movement; Masur, ever graceful under pressure, followed the old wartime British maxim by keeping calm and carrying on. Absurdity persisted, however, when yet more of the audience applauded after a thundering cadence midway through the final movement, eliciting audible chuckles from others in the crowd.

It is perhaps only fair to remark that part of the confusion may arise from the work’s atypical structure, ending as it does “not with a bang, but a whimper;” one has to wonder what Tchaikovsky, crippled by a lifelong fear of failure and rejection, would have thought of this audience’s reaction to his farewell composition.

Kate Stringer (MM in musicology from BU) is Research and Public Information Administrator at the Howard Gotlieb Archival Research Center at Boston University. In addition to her scholarly activities, she is a veteran actress, writer and director as well as a versatile mezzo-soprano.