Ok I finally can be certain what dirty beer lines taste like now. It is a flavor I have tasted at a few places over the years but until recently I could not pin point that it was dirty lines.
I ordered a Fat Tire at "le olive garden", very fancy place you should try it. Well the beer tasted nothing like Fat Tire, at all. Instead it had a very familiar taste that I instantly recognized because I have not cleaned my own beer lines for several batches and left them full of beer for an extended period of time.
Now I finally now what it tastes like and can start whittling down which bars I wont order from the tap anymore.