Through experience I have found that these caps take about 100 - 200 hours of play time to break in and sound their best. Now and then I get a customer who used solid state amps their whole life complain that these caps "seem rolled off". The top end frequency response is not rolled off but is so smooth to those use to the harsh top end of some solid state amps that something seems to be "missing" .. Yes - that solid state harshness is missing. I have heard other coupling caps like the Mundorf, Clarity, Auricap etc. and some (like the Mundorf's) were excellent sounding caps BUT - they are about $40 a cap about 5X times the cost of a Russian PIO cap.
Below is a link to a 2010 forum post that I made which shows what the inside of a Russian PIO looks like ...
Inside a Russian PIO cap