I also replied on that blog, as did others, so why just pick out Paul's reply?
The 'bitterness' as you call it is more like anger and disbelief about how they can replace all the original music (which, seeings as you obviously don't have a clue, is what the soul scene is all about) with an in house band singing the songs instead, which is sacriledge imo.
The entire play is based around Wigan Casino, where people danced to RECORDS, not a live band doing their own 'take' on those records.
You say the characters are fictional, the place certainly wasn't, nor was the music, and without that music those four actors on stage would not have been able to hold the interest of an audience for three hours, no matter how good they were.
The music is THE most important part of the play, and when that music is such an important part of the lives of everyone on here, what did you expect?