Believe it or not, I've gone through over 20 heads (across either the Aro tank or Protank II) that all tasted burnt or harsh with a silica bite. I've done everything to try and fix them. I finally got one to taste okay by adding my own flavor wicks, but though I'm getting a good amount of flavor, there's simply a taste of dirty dog hair coming through - I know it's still the silica influencing the flavor.
I personally think there may have been a big QC problem with these somewhere down the line. I learned my lesson with it, and am now only going to rebuild. I also tried the iClear 30, and it actually instantly burned on me worse than any Protank head, after letting it sit and absorb juice all night (with primer puffing it too.) I'd almost think the whole replaceable head system is a scam, or takes someone with super dull taste buds for them to be okay. But I know these types of things work - I once had a CE4 that worked as good as any dripper, all the sample devices at vendors seem to work okay, I have no clue what gives.