Hall & Oates sold a gazillion records and deserve every dollar. Their songs of the 1970s and 80s are pure pleasure; sun-kissed, smooth and mellow. It is a music of high noon, no shadows.

But what happens when twilight comes? Daryl Hall is 78. The partnership with John Oates, 77, has reached a messy end, with lawyers involved. Now he is on the road, under his own name, playing the songs of his gilded youth in a more tarnished age.

He strolls on to the Glasgow stage in a broad-brimmed hat and spends much of the set seated at a grand piano. His voice is not what it was in the same way as the Colosseum is not what it was: what remains is an interesting ruin. Where once his singing was a wonder of clarity, stamina and control, he now struggles. Sometimes, trying for high notes, he places a hand at the top of his chest, as if the effort is a strain. His slick six-piece band do a lot of heavy lifting on backing vocals. He has made an addition to the lyrics of Sara Smile that feels telling: “After all these years … time is talking to me.”

Yet there is beauty in this brokenness. Everytime You Go Away is a highlight because the feeling of bruised experience in his voice suits the subject of the song – a sad letting-go.

The big feelgood moments come from his sidemen. Shane Theriot’s guitar solos on Private Eyes and Rich Girl are undeniably cool, though not as cool as Charlie DeChant, a glitzy wizard with long white hair and a gold-sequined jacket who started playing with Hall & Oates in 1976. To see him shuffle forward and perform the insouciant sax break on Maneater is to witness a little moment of pop magic.

Such virtuosity magnifies rather than hides the diminishment in Hall’s voice. It feels at times like he is the weak link in his own great music. Perhaps he should do a Dylan: rework the songs radically so that he can do them justice. But would his fans go for that?

• Daryl Hall plays Royal Albert Hall, London, 19 May. Then tours the UK until 25 May