I cannot see a flaw in your argument; the EM would provide more preference satisfaction than RL.

However, I believe preference theory is built on air. Positive and negative experience is all that matters.

The glaring hole in the argument is that this assumes people know what makes them happy and what life they want, research however seems to show otherwise.

Even if PS theory was true it would not be rational to choose the machine, they'd likely stick with reality based on what they want. Plus people seem to find a strange joy in life not going the way they want sometimes.

Also altering one's preferences so they would prefer the machine is poisoning the well, any argument works if you permit brainwashing, so it's a moot point. In a sense it invalidates your point. "everyone would agree with me if they could be made to agree" doesn't really prove anything.

Ultimately your argument doesn't get at what the experience machine is about. The experience machine is arguing about whether pleasure and suffering are the sole factors of meaning in life and our decisions, and the PS doesn't get around that either. It's essentially just EP repackaged, and still assumes "welfare" (vague term) and pleasure are the sole good in life and should be maximized. But based on evidence while welfare and pleasure are part of it, they aren't the only values in life. Oddly enough, a lot prefer the unpredictability and messy nature of life, despite the harm it did to their welfare.

Heck I'm reminded of Frida Callo, that artist, who was in crippling pain her entire life but still had no regrets and lived "well". Or people who put themselves in mortal danger for the thrill even if they might not survive. Humans are weird...

So in a sense PS would argue against EM. Even if PS is true it would not be rational to choose the machine since not everyone would prefer that kinda life. Since we are talking about individual desires the word "rational" doesn't make sense. But also EM is to argue against Hedonism and that pleasure is the highest good, PS doesn't really support that since what someone prefers to live might not be pleasurable.

I'd also argue against preferences being experiences, they are judgments about experiences not experiences themselves.

You haven't made a case that welfare and happiness are the sole values for a meaningful life. It also makes the fallacious assumption that welfare is the goal of life, and given human behavior that fact is highly doubtful. It also assumes that living the life we prefer it part of welfare, but as evidence shows that's not entirely true. Turns out getting everything you want doesn't really lead to welfare, even a preferred life.

But also there is no objective improvement of welfare or measurement of it. That's a subjective judgment that cannot be measured in any empirically verifiable way.