I did lots of extract brews before I moved to AG, the twang seemed to come from inferior LME. After a few dodgy brews I moved across to the dried stuff DME/spraymalt and never had the twang again, a mate of mine has stuck with LME but only uses branded (Coopers) and never has it either. A few years ago one of the homebrew suppliers sold kits made up with unbranded LME and every one of those had the twang.
So stick to the dried stuff and you'll be fine.