Perhaps one could make a connection between Ferdinand's 'madness' when he discovers she has married and had children and his tormenting her with 'madmen'?
According to Legoius and Cazamian "Webster has a strange power of evoking shudders" and his uses madness as a device to raise shudders in the audience.
Madmen are employed by Ferdinand to torture the Duchess in her imprisonment and thus, to horrify the audience or the readers. Though Ferdinand has thought that the madmen would drive the Duchess crazy, it is quite the contrary which happens, for they, she says, help her to keep her sanity. The madmen provide a sort of distraction to her because just before their entrance, she has been presented with a dead's man hand and the supposedly dead figures of her husband and children.
The Masques of the madmen serve two purposes; firstly, it provides a comic relief although of a very grave and serious nature appealing mainly to the less sympathetic readers or audience. Secondly, il has a satirical purpose. The causes of madness are analysed by the servant and the analysis is directed against the excessive and peculiar craze which people of different professions develop.