A lot of “Deadheads,” as the Grateful Dead fans affectionately call themselves, either gravitated toward DMB or Phish when the band’s lead songwriter, Jerry Garcia, died in 1995, said Jeff Travitz, 61, a franchise development manager in Downingtown, Pa., who has taped over 100 DMB concerts. The bands are each unique, he said, but they filled the same “major void” to meet up with friends, tape shows and trade recordings.

“I don’t think too much about what we replaced,” Mr. Matthews said. Still, he understands the Phish comparison. In general, he thinks ’90s music critics dismissed improvisation, which, as far as he is concerned, is the only quality these groups share. “We all got thrown into the same category, even though we’re all different,” he said. “What do they call it? Jam bands?”

Whatever it’s labeled, fans have gone to great lengths for DMB. In 1998, the band launched the Warehouse, its official fan association, which allows members to pre-order tickets before the public, enter contests and access a message board. “I literally stole my mom’s credit card to join,” Mr. Roberts, the teacher, said. He now spearheads a Facebook group of about 850 DMB followers. Multiple times a tour, he will buy extra pit tickets, which cost about $50 to $150, with his own money and distribute them among the group at face value to combat scalping. (Mr. Matthews, too, laments the current business of ticketing: “I think half the profits that the ticket brokers make should be given back to the theaters, artists or charity, because they make so much money, and they’re really just scalpers.”)