I wish I would have sent the K-55's to Bob to be tested before I replaced the diaphragms.
There was an obvious improvement when I first installed the Allen drivers, but after the diaphragm change out on the K-55-V's, not so much. There is a difference.
They do curve ever so slightly different, with the Allen driver being just a bit smoother. I mean, it's not much, but it's measurable, so shouldn't be discounted.
I did use the system every night for almost a week before putting the Allen drivers back in.
Distortion: John Warren measured a K-55-M on a K-401 horn, and sent this to me via email ...
"K55M on K401 horn, voltage to horn terminals set for 110dB at fundamental, 1m from mouth of K401. No filter on horn. The FFT triggers off input sinewave. FFT bandwidth to about 48kHz. Second and third order make up the bulk of the distortion. %THD shown is average of 5 samples. %THD+N is a smidge over 0.5%..."
Al measured 1.5%, but used a K-55-V, and I don't know which one, or what condition the diaphragm was in.
I think I've decided that 30 year old diaphragms should probably be replaced.