When a big band take a big stage, the finer touches can get lost. For that reason alone, Hot As a Pistol, Keen As a Blade, the DVD of Elvis Costello & Allen Toussaint’s July 3 Montreal concert, is worth the price of admission. As those who heard the same tour (for the post-Katrina Verve Forecast collaboration, The River in Reverse) here in Boston at the Pavilion know, Toussaint’s piano, with its delicate New Orleans filigree, has a tendency to get lost beneath the guitar and brass. But here the mix is perfect: Toussaint’s lyrical keyboard and too-rare singing (notably his own “Who’s Gonna Help a Brother Get Further?” and “Yes We Can Can”) get equal play with “Big Sam” Williams’s cannon-fire trombone solos and brassed-up Costello hits like “Pump It Up.” And it’s a visual treat to see Toussaint slip onto the stage at the tail end of the opener, before the camera swoops down to his arched fingers, the polished black Steinway offering a mirror view. Costello makes a gracious frontman, chatting politics, and his “The River in Reverse” burns up the CD version. Bonus tracks offer Costello’s fleshed-out encore of “Allison,” interviews, and Toussaint in the studio recording “Brother,” eyes closed in soulful concentration.