Filtered is good so there's a bump in the right direction. Unfortunately, hardness of water is largely unaffected by filters so there's still the potential of excessive hardness in your water for lighter styles of beer. Generally this is associated with an extra "harshness" to the beer and not twangy, so this might not be the problem either. Do you know if you have "hard" water or not? I'm not certain how pH will play a factor in this considering you're not working with any grain. Certainly, a slightly more acidic final beer will have that extra bit of bite to it that is associated with light, crisp beers so pH may be worth looking into, but might not be the solution.
I also failed to suggest the DME itself as a point of twangy-ness. Sometimes, the extract can be the cause of those twangy flavors all by itself. This is where "freshness" matters, especially in lighter, more delicate styles. DME is generally more shelf-stable than LME, but when you can get good fresh LME then the results can potentially be better.
Do you have any idea on the "freshness" of the ingredients you're purchasing?