And big with hymn, commander of an host, The like was ne'er in Epsom blankets toss'd. Methinks I see the new Arion sail, The lute still trembling underneath thy nail. At thy well sharpen'd thumb from shore to shore The treble squeaks for fear, the basses roar: Echoes from Pissing-Alley, Shadwell call, And Shadwell they resound from Aston Hall.
The speaker describes as Shadwell rides into London victorious, as if he were the commander of a large army.
"Epsom blankets toss'd" is a reference both to Epsom Wells, a 1672 play by Shadwell, as well as to a line from another of his plays, The Virtuoso, which debuted in 1676.
Arion was an ancient Greek poet and musician. As the story goes, he was riding home on a ship when the sailors decided to kill him rob him for his wealth. He was permitted to sing one last song accompanied by his lyre and, after doing so, he jumped into the water—where he was saved by a dolphin who carried him to shore.
Apparently there was an actual "Pissing-Alley" in London during Dryden's day, where the locals would do their business (charming). Though, open sewers were commonplace everywhere.
"Aston Hall" likely refers to a Birmingham mansion, constructed several decades prior to the publication of the poem.
About thy boat the little fishes throng, As at the morning toast, that floats along. Sometimes as prince of thy harmonious band Thou wield'st thy papers in thy threshing hand. St. Andre's feet ne'er kept more equal time, Not ev'n the feet of thy own Psyche's rhyme: Though they in number as in sense excel So just, so like tautology they fell,
The term "toast" refers to waste in the river (ew). It's not an entirely flattering introduction for Shadwell.
St. Andre was a French master of dance, who choreographed Shadwell's 1675 opera libretto Psyche.
Dryden puns on the word "feet" here, referring to the feet of the dancer St. Andre, as well as to the metrical feet and rhyme scheme of Shadwell's Psyche.
We also get another dig at Shadwell's "tautology," his poorly-constructed writing.
That, pale with envy, Singleton forswore The lute and sword which he in triumph bore And vow'd he ne'er would act Villerius more. Here stopt the good old sire; and wept for joy In silent raptures of the hopeful boy. All arguments, but most his plays, persuade, That for anointed dullness he was made.
Who are these folks, you wonder? John Singleton was a court musician and singer, and Villerius is a character in Sir William Davenant's 1656 tragicomic opera, The Siege of Rhodes. He was a knight, and the grand master of Rhodes. The envious Singleton, the speaker implies, will be grand master of London no more, now that Shadwell is in town.
It can't be denied: Shadwell, in all his dullness, is the only man for the job. Everyone else is just playing for second place.