Even from a hedonistic perspective, 'Shut up and multiply' wouldn't necessarily equate to many beings experiencing pleasure.

It could come out to one superbeing experiencing maximal pleasure.

Actually, I think thinking this out (how big are entities, who are they?) will lead to good reasons why wireheading is not the answer.

Example: If I'm concerned about my personal pleasure, then maximizing the number of agents isn't a big issue. If my personal identity is less important than total pleasure maximizing, then I get killed and converted to orgasmium (be it one being or many). If my personal identity is more important... well, then we're not just multiplying hedons anymore.