The new PD5 IS the K-55-X, they are the same driver. Though the current driver has soldered terminals, it has a single phase plug and is identical to the original driver with the spring loaded terminals. Many are now assuming that since the new driver has the soldered lugs, that it is the same design as the driver that saw very limited use in the early 80s - it is not. The dual phase plug version of the K-55-V was quickly replaced by the EV K-55-M, which is basically the same driver from a performance perspective. I don't know how long Klipsch used the dual phase plug Atlas PD5-VH/K-55-V, but it wasn't long - maybe even only a year.The K-400 (as well as the 600 & 700) collapse the verticals and provide enough "squeeze" to get the response up to 5.8kHz. There is a sharp drop in response that begins a little after 4kHz, before quickly recovering and then rapidly climbing back up to nearly 6kHz. When Klipsch released the dual phase plug version, PK didn't think the improvement in performance would be noticed by most. I can hear it, but it's subtle. People saying that better performance can be had by dropping the crossover point are over simplifying things a bit, and are forgetting that though PK had numerous opportunities to do this, he didn't.
There is also a vast difference between a first order midrange filter that uses an autoformer and what Klipsch is currently doing -- a lower crossover point using steep filters with NO autoformer for the purpose of getting better phase behavior and producing better polar response. Changing a capacitor value and adding a coil doesn't do either of those things, in fact, the phase response gets worse, and the sonic signature is now dominated by the tweeter instead of the midrange, at least, that's what I hear.
I stand corrected as that was my assumption that the soldered pins terminals were the dual phase plug - out of 10 k55V I own -the best K55V is from the sixties , the serial number is under 3000 - push pin -