Hey pesky_wabbit stop chasing carrots and educate yourself.
To get an understanding about XO design try reading articles from the experts I mentioned. You are not arguing with me, you are arguing with Sigfied Linkwitz. Yes it's that Linkwitz of Linkwitz/Riley fame. His filters are probably more widely used than Bessel, Butterworth or Chebychev in crossovers.
You state: (ESR) "altering a fundamental characteristic of the crossover which affects frequency response"
For simplicity lets look at a first-order XO. A series resistance will NOT alter this basic function, the position of the XO knee or -3dB point is determined by the VALUE of capacitance, (not the value of capacitance plus ESR) The XO response has now been established and any resistance in series will merely attenuate the level to match the other drivers.
Now what about speakers with level controls for the tweeter? Are you claiming that when you adjust the level setting, which just switches different value resistors in circuit and here we are talking 10 or 20 ohms and not milliohms, that it is changing the XO point?
I consider neither your nor K Kantor's unsubstantiated bloviating worth hijacking this thread further.