UPDATE: The MMA has passed the Senate. See the full story here and read on to learn about the superstar letter to SiriusXM.

An array of superstar artists, producers and others, including Sir Paul McCartney, Don Henley, Mick Fleetwood, Steven Tyler, Katy Perry, Max Martin and Sia, have signed on to a highly publicized letter to the heads of SiriusXM urging them to reconsider their objections to the Music Modernization Act, which the missive (penned by songwriter Ross Golan) refers to as “the most consequential music legislation in 109 years.”

If SXM doesn't budge? "We will boycott," the artists proclaim.

The basis for SXM being the lone holdout in the biz? Pre-1972 royalties, which the digital-radio giant says gives the terrestrials an unfair advantage. But given the number and resolve of the MMA’s advocates—and its support by an overwhelming bipartisan majority in both houses—this may be a short-lived stand. If not, MMA supporters have indicated, it’s all-out war on the Liberty Media-owned satellite behemoth.

Expect further action very soon on Capitol Hill, as the bill has racked up 80+ cosponsors.

Meanwhile, biz insiders and artists alike are acknowledging the shrewd leadership of NMPA boss David Israelite in marshalling the troops for this battle and hitting the requisite pressure points. Passage is expected to lead to a sizable revenue bump for rights holders.