These caps are extremely transparent in the many uses I did. 
Both high voltage 600 and 1000 volts 

I’ve used them in dc blocking and seem to have a neutral sound 

has anyone tried these in Amps and pre amps 

I’m curious to know what others think 

I’m not a fan of anything oil as I feel they color the sound. 
In addition in stat amp builds 400 vdc seems on the edge of max range 

why don’t builders use 1000 vdc rated caps ?

 

ABD0000C255.pdf

Panasonic makes great film caps. I've used many different types for many applications. When used for audio coupling, my experience has been that higher voltage ratings tend to sound better. Look for caps designed for high frequency, high voltage, high current circuits (like PFC or resonant power circuits) and you'll usually find good construction that has nothing objectionable to audio.

The ECWFD series you linked has copper-clad steel leads. I tend to look for copper leads on principle, though most datasheets will just say "wire leads". According to a list I made many years ago, Panasonic series ECWH(V), ECWH(C), and ECWFG have copper leads. Series ECWF(L), ECWF(A), ECQUA have copper or copper-clad leads. I think the leads are more likely to be copper with higher voltage ratings.

The only slightly annoying thing about the Panasonic caps is that they'll sometimes throw out a non-standard lead spacing (17.5mm, why?), but I don't think that will matter to most people.