Hello. Probably one week, most likely two or three...I had the same experience on purchasing a new ARC SP9MkIII pre-amp some years ago - having heard a used MkII with great results, I could not believe how 'awful' the new MkIII sounded straight out of the box - after a week it was a good, and after a month it was perfect. I've been recently auditioning tube power amps as potential replacements for an ARC D125. The shop, Audio Connection in Sydney, Australia, has a great choice, so much so that their demos are virtually unused - this meant a Jadis Defy 7 and an ARC VS110 both sounded 'awful' in exactly the way you've described. Congested and harsh. The store suggested a few weeks before they would at their best - in the meantime I'm meant to 'use my imagination' during auditions! Hooking up used amps to the same system saw much smoother results. Good luck. Rob.
