After all these years, 3 Doors Down are still writing soundtrack-ready, doom-and-gloom scorchers about matters of life-and-death urgency. It’s a profitable formula, and all their albums since their 2000 hit single “Kryptonite” have charted reliably. It’s been a while since Staind, who open for 3 Doors Down at their 6:30 p.m. Marcus Amphitheater show tonight, released their signature 2001 single, but since that hit, the one-time Fred Durst prot�g�s have continued their shift away from alt-metal and toward Alice in Chains-styled buzz ballad. The night’s other openers Hinder, scored a No. 1 single of their own in 2006 with “Lips of an Angel,” a power ballad that would make Aerosmith jealous.