The judges comment about a mineral taste may have lead you down the wrong path. In my experience, water chemistry makes relatively subtle changes in your beer. If you have a serious flavor flaw then I would bet that it a fermentation problem. Checking your thermometer was a good move.
To isolate the problem you might consider brewing a very simple, medium gravity ale and concentrate on getting nice fresh malt favors. You can't exactly do a PM SMASH, but that is the right idea. You can pretty much eliminate yeast problems by using dry yeast, too. In the old days dry yeast was dreadful, but today's S-05 is very good. The only good reason to use a liquid now is to get unusual strains that are not available dry.
A four pound mash plus three pounds DME, two or three hop additions, a packet of yeast and you are good to go. Try to get it into bottles ASAP and skip the dry hopping. Strong fermentation and freshness are key.
Are you using an oxygen stone? If not then get one, it makes a huge difference. Got to keep the yeast happy. If the yeast ain't happy, ain't nobody happy!
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