Sometimes described as a 6-foot-tall scowl, Sergei Rachmaninoff seemed perfectly genial while playing through what would be his last major orchestral work, Symphonic Dances. A great pianist as well as a composer, he played his own keyboard-only version with vocal counterpoint.

His audience was a small circle of listeners in the home of then-Philadelphia Orchestra music director Eugene Ormandy, who premiered the piece only weeks later in early 1941.

Now a popular calling card for the Philadelphia Orchestra at home and especially on tour, Symphonic Dances can be heard in this pre-premiere play-through on the Marston label — one of several new classical sets that allow listeners to eavesdrop on long-passed musical giants.

Discovered in the Ormandy archive at the University of Pennsylvania, Rachmaninoff Plays Symphonic Dances probably wouldn’t have been viable for release until recent technological improvements came along. The pianist is “off-mic” — probably recorded without his knowledge — and the sound quality still requires ears willing to meet it more than halfway. But at least you don’t have to deal with Rachmaninoff’s cigarette smoke.

Meanwhile, is issuing live Boston Symphony Orchestra recordings heard in astonishing high fidelity from the late 1940s, again in recordings made secretly: A radio producer hid a microphone in a ventilating grill and caught such reputedly stone-faced legends as violinist Jascha Heifetz and conductor Serge Koussevitzky performing with an exuberant freedom and excellent sound not often heard elsewhere in this period.

The legendary Arturo Toscanini sometimes could be his own worst enemy by allowing the ultra-dry recording-studio sound quality that afflicts his NBC Symphony Orchestra recordings. Now, Pristine also has his 1939 Beethoven symphony cycle — in all of its unmediated fierceness — with such vivid sound that these performances could haunt you in your dreams.

But the talk of the summer among classical collectors is the lavish box set Wilhelm Furtwangler: The Radio Recordings 1939-1945, produced by the Berlin Philharmonic with 22 discs of performances heard in unprecedented clear sound from inside the crumbling Third Reich. More such sets can be expected from any number of sources as archives are reexamined and private collectors allow their holdings to reach eager ears in the larger public.

Why all this old stuff now?

Historic recordings have made continual advances in the digital age. And now classical listeners who have resisted their substandard sound quality — like moviegoers who won’t see anything in black-and-white — no longer have that barrier. Also, greater appreciation for historic value overrides the fragmentary state in which some of them survive, such as the Rachmaninoff recording.

Previous generations didn’t love the Symphonic Dances anywhere near as much as modern audiences do. Ormandy was ambivalent about the piece, and Rachmaninoff was even more doubtful about Ormandy’s performances of it. From this uncertain start, it’s not surprising that the piece has drifted into suaveness, as with the ingratiating performances led in years past by Charles Dutoit in Philadelphia.

Not so in Rachmaninoff’s playing, which has far greater freedom and flexibility than in his famous commercial recordings with the Philadelphia Orchestra. In this casual, fragmentary setting, the famously lyrical saxophone solo in the first movement has a marvelous give and take that reminds you how the composer once considered having that part sung — by Marian Anderson, no less.

Album cover of "Rachmaninoff Plays Symphonic Dances: Newly Discovered 1940 Recording" from Marston.
Album cover of "Rachmaninoff Plays Symphonic Dances: Newly Discovered 1940 Recording" from Marston.

The ghostly second movement also has a revelation in the way Rachmaninoff plays the dance rhythm with less-graceful grotesqueness, which is far more interesting.

First-movement tempos were initially too slow. Conductor Dmitri Mitropoulos had been sternly corrected by the composer prior to his 1942 live recording of Symphonic Dances with the New York Philharmonic that’s included in the Marston set. The results feel welcomely gritty and rhythmically emphatic.

For all his sympathetic treatment of other Rachmaninoff works, Ormandy seems not to have gotten that memo in his 1960 Philadelphia recording with the Philadelphia Orchestra, which drags dutifully. Though Rachmaninoff’s recording isn’t a prescription for how the piece should always be done, it’s a clear point of reference from an irrefutable source.

Furtwangler for a new century

The Furtwangler set has a more lurid history. Often accused of Nazi sympathies for staying in Germany throughout World War II, Furtwangler in fact maneuvered away from state functions and used his influence to get numerous Jews safely out of the country.

His own exit strategy — taking over the New York Philharmonic — came too late: He was already guilty by association. Later, as defeat loomed in Germany, Furtwangler was induced to make many radio recordings found in this new set.

In fact, they were coded protests. His mercurial, highly personal manner is often heard as a reaction to anything from Kristallnacht to the Battle of the Bulge. His 1942 Beethoven Symphony No. 9 has always seemed angry; now heard in optimum sound, it expresses rage.

The 9th symphonies of Bruckner and Schubert, always intense, are now harrowing. When his concertmaster, Erich Rohn, played the Beethoven Violin Concerto, he did so with a cadenza by Fritz Kreisler, an officially banned Jewish composer, right under Nazi noses.

The high-quality tapes were then stolen by the Russians and later issued on LPs, sometimes with bonus tracks of Russian works not conducted by Furtwangler at all but implying that he was welcoming the liberators.

Whenever possible, this newly produced set uses original tapes (returned from Russia), all remastered with Super Audio CD. Not everything is up to modern sound quality, but much is, allowing you to hear the ambience of the concert halls that were soon to be bombed out of existence.

The Berlin Philharmonic was on its third venue in January 1945 when Furtwangler’s last wartime concert in Berlin — the Nazis had turned on him and he was preparing to flee — was interrupted by a power failure, during which the orchestra kept playing Mozart as long as their memories held up.

The concert resumed with the Brahms Symphony No. 1. The recording of its final movement (the only surviving part of that concert) encapsulates extremes of grief and exaltation.

Germans were in retreat on both fronts, with the Russians on their way from the east. This is Brahms at the end of the world — played and heard with a sense that this might be the last music anybody in the that building would ever hear. It is the final gasp of pre-World War II German culture. Furtwangler and the orchestra hung on every note.

What does any of this have to say to modern times? History is like a Rorschach inkblot test, so you never know.

One thing that the Furtwangler set explained, for me, was the troubled tenure of Christoph Eschenbach at the Philadelphia Orchestra (2003-2008). “Some aspects of Furtwangler’s performing style … ,” states Richard Taruskin in his album notes, “would damage anyone’s career.”

And Eschenbach had many Furtwanglerian aspects, with flexible, meditative tempos, performances that didn’t always reflect what had been rehearsed, and ensemble train wrecks as a result. Taruskin could’ve been quoting the elevator conversations I had with the disgruntled Philadelphia musicians during the Eschenbach tenure.

Yet such performances open your ears to greater possibilities of meaning and expression in the supposedly static world of classical music. They represent a lost world.