Any string will growl (within reason, I did struggle with a set of Pirastro...never again!). Spirocores growl more than Dannys signature set (I use his strings for A D and G and have a medium Spirocore on the E).
I'm the the guy that posted the story about playing Dannys Bass, he told me off for having a stupidly high action! Victoria's action was pretty low, he was twice as loud as me when we played unamplified and he wasn't really digging in with his right hand. The reason he raised the action at the nut when he practised was to build up left hand strength, his aim has always been to produce a fingered note that had as much tone and clarity as an open string.
Pickup wise, he's been using a blend of two for years, don't know if the Underwood still gets used, ( he defected to shadow for a bit...) but I do know he has a Highlander piezo ribbon pickup inlaid into his bridge about a half centimetre below the strings, he told me its a guitar pickup....
He was using a Ravenlabs PMB2 preamp/blender made for him by Steve Rabe of SWR fame, but when he started using the AER Basscube, he ditched it.
Danny told me it was years before he had any kind of amp and a couple of decades before he cracked the pickup conundrum, so by the time he'd gone down that road, his sound was already fully developed.......
He's on facebook and is very accessible and free with his information and time to anyone, give him a shout.