Hi Nigel,
I haven't played vinyl records for a while and can't even remember where they are (I kept them somewhere). They were recorded and made through many caps, some probably electrolytics.
I don't think my CDs were recorded through electrolytic coupling caps.
While testing the 1st few versions of my mod-4 FM transmitter, I saw low frequency distortion on my 'scope from an electrolytic cap as a pre-emphasis maker. I guess the circuit's rising frequency response emphasized the distortion. My circuit uses a very low supply voltage and the distortion from electrolytic caps is increased with low bias voltages. When I replaced the electrolytic with a poly one, there wasn't any visible distortion.
10% distortion is difficult to see but plainly heard.
I built only a single one, and didn't try a replacement elco, so maybe Mr. Murphy came by for a visit.
I just finished listening to a series of tones from the web, some pure and others with 1% second harmonic distortion added. The 1st time I played them I was distracted by my wifey asking me to turn it off (I should have used headphones), then each time I repeated it I got a steady tone.
My 1st amp had tooobes (valves) and when I took it to the store for their MacIntosh Free Amp Test, its distortion was about 15%! It crept up as the output tooobes mis-matched themselves and I didn't even notice!
My latest amp is a fairly inexpensive Japanese one (Yamaha), has "High Dynamic Power Low Impedance Drive Capability", very low distortion and only a single coupling cap (poly input) in its DC-coupled design.
I used poly coupling and frequency-determining caps in my bass-EQ circuit. My complete system sounds excellent!