"It must be really satisfying to know that you are singing some of the most difficult choral music ever composed - and that you are singing it really well."
The music in question was Bach's B Minor Mass: pillar of the repertoire and still some of the most intimidating vocal music ever written. Clocking in at about two hours (give or take several minutes depending on stylistic whims), this is a score that demands a huge amount of mental and physical stamina to pull off.
It is set high for the voices, especially when performed at modern tuning rather than the lower Baroque pitch, and is infused with the all the technical might of Bach's experience, from plainchant to staggering counterpoint, to florid operatic ornaments to feisty dance rhythms.
A degree of mystery shrouds its composition - why did the devoutly Lutheran Bach pour so much energy into a full Catholic Mass setting? - but what seems fairly clear is he did not expect it to be performed in one go. Nowadays complete runs are standard procedure, but that should not detract from the awesomeness of the experience for audience and performers.
After tea break at Sunday's rehearsal, Chorus members are reconvening to tackle the mighty Sanctus. They open with a tremendous, warm D-Major sound, beefy basses supporting vibrant inner voices and ringing sopranos. This choir has become a different beast in recent years. It is leaner, cleaner, more buoyant, better blended, more powerful than it used to be.
It is still an amateur ensemble made up of 55 members who meet on a Monday night after their various day jobs. But its calibre is beginning to stand up next to its world-class orchestral partner.
The change has been ushered in by a new chorusmaster, Gregory Batsleer. When the laddish Mancunian arrived in 2009 and found a Chorus that lacked the vital spark to match the SCO, he reauditioned the entire ensemble, established annual assessments and hired a no-nonsense vocal coach, Pat MacMahon. It was a shock to the system for many who had been in the Chorus for decades, but few who hear the group now could contest that the results have been worth it.
"It's like choosing a good football team," says Batsleer (a card-carrying Manchester City fan). "If you can get a better striker who is going to score you more goals, you have to go for that player."
Above all, what he looks for in auditions is a voice "that is free. Sure, the singers need a degree of musical vocabulary - in rehearsals we use quite highly detailed harmonic language that they have got to be able to keep up with it - but that can come, like learning a foreign language by jumping in at the deep end. Basically, we need to make a core sound that is blended and flexible, like the orchestra, and one that can respond when the conductor puts his foot down and says go."
This week's B Minor Mass has gained the added challenge of a last-minute change of conductor. With the SCO's associate artist Richard Egarr knocked out by flu, Cohen was drafted in late last week. He is a luxury back-up option - artistic director of the period-instrument ensemble Arcangelo; associate conductor of Les Arts Florissants - and his easy authority with the Chorus is evident throughout the rehearsal. Batsleer sits on the sidelines, watching like a proud parent.
"As a chorusmaster you have to get to a position where the Chorus fundamentally trusts you," he says afterwards. "Only this year - my fourth year in the job - have I really built that."
It is about much more than the technical stuff. The choice of metaphors he uses to evoke certain sounds; the ability to mediate between guest conductors and the Chorus; judging the right balance of energy, banter, authority and compassion.
Performances are "like watching your football team at a cup final", he says. "A thrill, but never straight-forwardly enjoyable. I am so acutely aware of all the building bricks."
Batsleer grew up in inner-city Manchester and sang as a treble in the (now-defunct) Manchester Boys Choir. As a teenager he "obsessed over Palestrina" and played Allegri's Miserere on repeat, but he never let on his passion for choral repertoire to his classmates. "They knew I would go away with the choir for a couple of weeks sometimes, but they did not know how into it I was. I now regret making it a secret. If I ever have children, I will tell them to go to school and tell everyone how much they love classical music."
He went on to study music at Princeton, was offered the post of director of the Hallé Youth Choir a year into his undergraduate and transferred to the Royal College Of Music to finish his degree as a bass-baritone while juggling his new job. With the Hallé stamp on his CV he was off, and he swept up the SCO job as his first full chorusmaster position.
"There are so many talented young conductors in the UK," he says, "but the route into becoming a chorusmaster is less obvious. Up until about 20 years ago almost all chorusmasters were organists, but the culture is changing and there are a lot of people with a lot of fantastic energy in the business."
Batsleer mixes his work with the Hallé, the SCO and the National Portrait Gallery (he directs the newly-formed Portrait Choir) with non-classical projects: sound design for plays, producing albums for singer/songwriters, preparing choirs for rock bands. He says the range is essential for keeping his musical ideas fresh.
"When I take a rehearsal with the SCO Chorus, I still feel like a child. Afterwards, I'm usually awake at 1am with excitement of it." Listening to the Chorus now, I would guess many of them feel the same way.
The SCO Chorus sings Bach's B Minor Mass at the Queen's Hall, Edinburgh, tomorrow and City Halls, Glasgow, on Friday