Imiela, whose victims included a 10-year-old girl, began playing at Wakefield in 2008.
At the time, another key band member was sick child killer Dominic McKilligan.
He is serving life for raping and killing 11-year-old Wesley Neailey in Newcastle in 1998.
The two lags meet each week in the jail’s chapel for an hour-long practice session.
The core group of a dozen men includes other killers, rapists and paedophiles.
Both Imiela and McKilligan are said to have taken turns as the “star” singer in the group.
The source added: “That’s why they were given the name the Monsters of Rock.”