Vienna Philharmonic/Jansons/Thielemann, Royal Festival Hall, London

Imagine settling down to a concert when suddenly you notice a manky manikin dressed in a flea-bitten jacket sitting close by. That's what happened on last Monday night at the Royal Festival Hall. At least one lady leant visibly in the opposite direction. "Don't like the look of that," she mumbled. An announcement explained that pre-refurbishment acoustical tests were in progress, with manikins positioned at key locations and an eerie sequence of test signals that would have had Edgar Varèse in ecstasies. But the tests didn't end there. Both Monday's and Tuesday's Vienna Philharmonic concerts featured conductors tipped for the top job at the Amsterdam Concertgebouw. I hope the governing powers were listening carefully.

Mariss Jansons took care of Monday. With no baton and an open score that he barely looked at, Jansons traced a patient opening Andante for Mendelssohn's Scottish Symphony, gesturing sparely, bringing out the radiant Viennese horns, and urging on the agitated main allegro without pushing too hard. The scherzo was more busy than elfin, the Adagio best where the violins or cellos took the lead, and the finale trim but unremarkable. Great music, good playing, clear thinking (albeit with the occasional adjusted dynamic) – and that was about the measure of it.

Jansons's lean, "plain Jane" approach suited the imaginative tailoring of Haydn's Symphony No 97 in C rather better. And when did you last notice sul ponticello strings in the Adagio ma non troppo movement?

Last up on Jansons' official schedule was Stravinsky's 1919 Firebird Suite, a little loud and plodding to start with, and I've heard Kashchei's "Infernal Dance" sound more reckless, but the transition from the Lullaby to the Finale was magical. Always a canny choice for a closing item, Stravinsky's finale psyched up the capacity audience for two encores, Dvorak's 15th Slavonic Dance, which was fast but faceless, and a rousing account of Prokofiev's "Death of Tybalt", in which Prokofiev's metrical timpani death throes were bizarrely distorted.

I'd expected even more musical distortion the following night, when Christian Thielemann took to the rostrum, but was very pleasantly surprised, at times even inspired. Once a shameless Furtwängler clone, Thielemann has become his own man, and he conducts entirely from memory. He stirred a vigorous baton through the main body of Mendelssohn's A Midsummer Night's Dream Overture, toing and froing between divided violin desks (Jansons sat them next to each other) and conjuring a saintly slow-motion coda. After the savage brass motive that opens Richard Strauss's Die Frau ohne Schatten Suite, Thielemann intensified the score's blossoming string lines with the skill of a true master. True, Strauss's Ein Heldenleben ventured momentarily over the top towards the end of a very loud "Hero's Battlefield", but was otherwise played magnificently with disciplined affection, especially by violinist Rainer Honeck, who, in "The Hero's Courtship", balanced sweetness and caprice to perfection.

On a bad night, Ein Heldenleben can sound banal, even tawdry, but not on this night. I sensed a great conductor in the making, shedding old excesses and putting the music first. The encore – just one this time – was from Strauss's Capriccio, brief but beautiful and sure proof that the orchestra was following Thielemann to a man. And, just occasionally, to a woman. I counted four on Monday. The previous night, there had only been one.