essentially an Epicurean ought not plug oneself into this machine that promises pleasure,
I think Melkor's summary is correct, but in reading it I think it's important to understand that we are making presumptions about the pleasure machine scenario that are very important. Is it inherent in the scenario that the issue with the pleasure machine is lack of variety? (IE, that there are pleasures we would want that the pleasure machine would not provide?)
If we presume that that is the case, then yes an Epicurean would not choose the pleasure machine.
However I do think we need to be careful to state that detail of reasoning, because if we agree that pleasure is at least somewhat subjective, then it might well be acceptable for an Epicurean to decide that the pleasures provided by the pleasure machine would provide him a better life than some other alternative, and choose the machine.
Stated differently, "if" we found that we derived great pleasure from staring at a candle, and "if" we could postulate that we had circumstances which allowed us to sustain that pleasure indefinitely, would Epicurus say that Nature requires or guides us to reject that option?
Remember PD10: "10. If the things that produce the pleasures of profligates could dispel the fears of the mind about the phenomena of the sky, and death, and its pains, and also teach the limits of desires (and of pains), we should never have cause to blame them: for they would be filling themselves full, with pleasures from every source, and never have pain of body or mind, which is the evil of life."
It seems to me that this is a reminder that our problem with being a worm or staring at a candle is not that being a scientist or an astronaut or a great artist provides "intrinsically" superior or more intense pleasure that is choiceworthy for everyone because of its superiority or intensity. It seems to me that as a practical matter, and from the "point of view of Nature," staring at a candle is not a human-sustainable lifestyle, while pursuing a career as a great artist or astronaut or scientist is or can be both intensely enjoyable and sustainable. So while I don't think we can say that the candle-starer is "objectively" wrong to pursue a life of that kind of pleasure, at least for myself I can and do state emphatically that a life of five years of being an astronaut is something I would choose over 50 years of staring at a candle.
But to repeat one more time, "IF" we stipulate that the person staring at a candle is indeed experiencing pleasure, then we may not personally be impressed, but there's still no god or realm of platonic forms justifying us in saying "that person isn't living right."