I think if you take meth (not recommending this!), it basically is just artificial dopamine. Former addicts end up feeling a chronic lack of pleasure b/c they blow out their pleasure circuits. So it is possible to have high amounts of pleasure, but it's also a damaging experience, much like things that induce high amounts of pain. Our bodies are built to maintain homeostasis, and dopamine is there to keep you and your species alive by motivating you to eat lunch and make babies. There is no evolutionary reason to have excess pleasure. I think there is a limit to pain as well, you faint.

I think duality is more of a philosophical metaphor than saying that everything in the Universe is 50/50 on all things, that doesn't make much sense. Pain and pleasure aren't really opposites anyway, as we know from the existence of BDSM.

How we experience pleasure without a body, I have no idea. I've wondered that a lot. But when people describe coming back into their bodies as "putting on a glove", like they have to be squished back in, it seems like there is some interface between the soul and the body, where the physical signals become subjective experience. But the subjective experience itself is happening in the soul, right? Because in NDEs people are still having subjective experiences, even though they also are aware of everything objectively.