I'll try to comment briefly on various Internet posts about why VMPS is so darn peculiar in so many different ways. 

If flat amplitude response is your primary concern, high order slopes and drivers alternately in inverted polarity is the way to go. Response is considerably flatter in the crossover regions and there is less lobing with frequency. 

I use first order networks and in-phase drivers because that emulates live music best. I add "protection" filter poles outside the passband to protect the mid panels and treble ribbons from damage. These legs are placed well away from the crossover point and have no sonic impact. I can remove them and see no change in sound quality or measurements. The only result of not having these "protection" parts is that the panels buzz and the tweeter diaphragm stretches without them. 

We use pots because they allow a 1/20th of 1dB change in level to be effected by the user. Often such a change is all that is needed to tame hot trebles or a forward midrange. A fixed value does not allow the user to adapt the music to his tastes. Also, resistors are not made with sufficient tolerances to give me the precision I feel necessary for level changes. Also, fixed parts change value with age and there's nothing you can do about that. The pots change too, but all you have to do is move the wiper over one winding and you're back in business. Fixed values are invariably determined by measurement, i.e. whatever meassures flattest with your mic and test gear and environment. However, bass below 200Hz is boundary-dependent; fixed values do not allow for changes in room or placement. You're going to be wrong virtually 100% of the time, no matter what you think your measurements tell you is best. 

Room correction is best effected passively, with bass traps in the corners and absorption behind and beside the speakers. Digital correction and EQ are fine if you listen only at the mic position. Since you are using a single mic and have two ears spaced about 8" apart, this may give strange results. 

High order filters have poor transient response and ring like bells behind every note. If you can handle that you are less sensitive to such garbage than I am. 

In room measurements are worse than useless and mislead the audiophiles into making huge corrections that can be simply measurement artifacts like floor bounce and late arrivals. I've been doing this nearly 30 years and have learned the hard way. 

The PR tuning must be done in very small increments. While amplitude response changes little with tuning, harmonic distortion changes greatly. I published curves last year, showing 10% THD at 20Hz on the RM/X with the PR untuned and 1% THD with about half a gram of putty removed. The sound becomes dramatically clearer and the bass response subjectively (emphasis here) summes up when the tuning is done properly. I invented this bass system in 1979 and was highly gratified to see a Klipsch speaker adopt it a few years later. Imitation and flattery, you know. 

I make improvements as I see fit. These upgrades are available at low cost or no cost to anyone interested. I am very concerned that all VMPS owners have the best possible sound at home, since this keeps me in business. I go to extraordinary lengths to make this happen. 

My 58-year-old hearing is unlike yours, I'll wager. I design and listen in an ideal acoustic enviroment (31' LEDE), yet with the adjustments provided, I can go into a completely untreated room and get near state-of-the-art sound from modest equipment and a tight budget, by following the directions set forth in our flyers such as "User Adjustable Bass Damping". 

In my book, textbook filter design, driver selection and layout, and measurement techniques are useless.

Brian Cheney (2005)