My tuppence worth...
I've returned to kit brewing to supplement my stocks between the rare AG brew days. When I first started brewing kits, before I jumped to AG, I did find that some had that 'twang'. These days however, they don't, which could be because I now simply add Campden tablets to the cold 'top up' water, which i didn't do previously. Maybe that's just luck, improved brewing techniques/equipment, or type of kits that I now brew, but it appears to be working.
In the past year, I've done two Premium Lager kits from The Range and a Muntons Connoisseurs Bock. No twang, and they have been a roaring success with friends and family. It is worth noting though, that my tap water is soft, and I use LME (Coopers Light) instead of DME, sugar or beer enhancer, and I've been swapping out the supplied yeast with my own preference.
So, roughly half a campden tablet to my cold top up water. No treatment to my boiled water.