As Spotify continues to clear the decks for an expected public offering this year or next, it has hit another potential roadblock.

In May, the company proposed a $43 million settlement in an effort to resolve any claims that the company hasn’t paid proper mechanical licenses for song compositions. That deal is still awaiting a judge’s approval.

In a court filing dated Tuesday, an unusual collection of musicians including Tom Petty, Kenny Rogers, Weezer frontman Rivers Cuomo, Sonic Youth’s Kim Gordon, David Cassidy, Black Keys frontman Dan Auerbach, actor (and occasional classical composer) Anthony Perkins and many others slammed the $43 million proposal (the news was first reported by The Hollywood Reporter).

“The Settlement Agreement is procedurally and substantively unfair to Settlement Class Members because it prevents meaningful participation by rights holders and offers them an unfair dollar amount in light of Spotify’s ongoing, willful copyright infringement of their works,” the filing reads in part. It also claims that after attorney fees, the $43 million is down to $28.7 million.

“According to its own publicity, Spotify has about 30 million songs,” it continues. “If as few as one-quarter of those songs, 7.5 million, were unlicensed then, taking the $28.7 million left in the Settlement Fund after attorneys’ fees are paid and dividing that number by 7.5 million songs, the result is a settlement payment of $3.82 per infringed song. As the Court is well aware, Spotify faces potential liability of up to $150,000 per infringed composition for willful infringement and $30,000 for nonwillful infringement, plus attorneys’ fees and costs. While a discount of some amount is to be reasonably expected as part of a compromise settlement, the discount potentially afforded Spotify in this case is a 98.7% discount for nonwillful infringement and a discount for willful infringement so close to 100% as to give Spotify a practical free pass on willful infringement.”

Less than $4 per song is a strong number to rally the troops, and not insignificantly, the first artist named in the filing is singer-songwriter Melissa Ferrick, who with Cracker frontman David Lowery has been one of the lead instigators in the royalty battle against Spotify. Their class-action suits were settled by the $43 million fund in May — but apparently, not permanently.