Last week, the troubled Broadway musical "Lennon" met its toughest critic yet — Rolling Stone founder Jann Wenner (search).

A close friend of John Lennon's (search), Wenner was invited by Yoko Ono (search) to the dress rehearsal of the show last Wednesday.

He was spotted slumped in his seat throughout the performance.

The next day, production sources say, he fired off an e-mail to Ono that was highly critical of "Lennon," which tells the ex-Beatle's life story through his music and lyrics — with several different actors, male and female, white and black, playing Lennon.

Wenner's response has apparently been a wake-up call.

Until last week, Ono didn't think "Lennon" needed much revision, and she shielded Don Scardino (search), the director and writer of the show's book, from criticism that his concept wasn't working.

But now, sources say, she realizes the show needs a lot of retooling before its Aug. 4 opening.

Alan McKeown, the lead producer of "Lennon," declined to comment yesterday on Wenner's e-mail.

But he told The Post: "Yoko's views and ours now completely correspond. We are changing the show on a daily basis, and it is my hope that when we do open on Broadway, we live up to the legacy of John Lennon."

Although Ono, Lennon's widow, is not a producer of the show, she has a lot of power because she controls Lennon's image and his estate.

She's been instrumental in the development of the show from the beginning.

She was with the production in San Francisco, where it opened to negative reviews, and has attended nearly every preview in New York.

The 72-year-old is easy to spot in the theater because she always wears sunglasses and a big floppy hat.

Among the musical's problems, sources say, is the lack of a clear narrative.

The multiple Lennons only add to the confusion.

The tone of the show is another problem."Lennon" is said to resemble a gooey hippie happening, sort of like "Godspell," the 1971 hippie show in which Scardino played Jesus.

Not surprisingly, given Ono's involvement, the show treats her with reverence.

At one point, the Lennon character says: "If all this had to happen to me to meet Yoko, it was worth it."

Bob Eaton and Brian Hendel, both writers and friends of Ono (Hendel once edited the Yoko Ono newsletter), have been brought in to work on the show.

Tommy Tune also stopped by — he's a friend of Ono's, too — and made some suggestions.

McKeown admits that putting "Lennon" together "has been a challenge. I thought the songs would really drive our narrative along. I was completely wrong. You need a book. And now we are putting our hearts and souls into the writing. Yoko is completely supportive. 'Lennon' is getting better and better."