Great to hear the news about the latest album / tour Steve.
My suggestions for what you could sing with Jools are

Otis Redding - Try a little tenderness
Smokey Robinson - I second that emotion
Bob Dylan - Like a rolling stone

Also from Motown, there are two songs from the Four Tops which would suit Steve's voice perfectly,
"Reach Out I'll be there" and "I can't help myself"

MY MOTOWN SUGGESTIONS FOR STEVE AND JOOLS ARE AS FOLLOWS....
the night,frankie valli and the four seasons
david ruffin,you can come right back to me
edwin starr,s.o.s.(stop her on sight)
the elgins,heaven must have sent you
r.dean taylor,indiana wants me
the isley brothers,behind a painted smile
stevie wonder,pastime paradise, or any track from the magnificent 'songs in the key of life' album...pure genius.
jimbo
chesterfield

...just thought of another one, how could I forget?
The Righteous Brothers - Unchained Melody

How about a reprise of ' I Wish It Would Rain' ? Steve's voice and Jools' piano would blend really well.

Or The Drifters 'Sand In My Shoes', Stevie Wonder's 'Yester Me, Yester You, Yesterday'
In fact I'd love Steve to record a covers album of Motown, r&b and soul, ain't gonna happen, but can still dream, even pushing 50 !

Seb, 'Unchained Melody' was an occasional live cover, back in the late 80's/early 90's.

And fantastic it was, too !