My UltraFlo weldeds have held up perfectly and still after all these years no drone at any rpm. They have been running behind 2.5" Longbranches w/an X-pipe. And now behind the long tube Dougs and they sound awesome, IMHO. They don't sound anything like glass packs! I know after I bought mine they came out with something they called roving fiberglass packing but that may have been on their std turbo muffs.