Update - pulled apart the sub finally. Definitely the amp is toast. Luckily, hooking up another amp, driver performs beautifully! Talked with HSU, they can either repair the amp or can send a mk4 to replace, latter a bit more expensive. I'm up in Canada so a bit of a pain to ship/pickup/customs paperwork (I assume no duties as it's a repair). The Dayton SPA-250 I can get in Canada for about the same as from US with exchange/gas to pickup across border. Am likely going with the Dayton based on reviews and ease of purchase. I did however see an inuke 6000 come up on craigslist for about 400 Cdn (about 150 more than the Dayton). Would that be way overkill for the HSU or would it possibly eek out more performance? Would definitely allow me to make a giant sub out of my second row riser - thinking about 3ftx8ftx15inches - would a huge box like that work well with the 12" hsu driver or is it just begging for an 18" or 2?