Yeah, exactly as our-Ray mentioned. It's just a soft silicone rubber mat to stop equipment sliding about on the bench. Every so often I have to vacuum clean it to get bits of wire and solder blobs out of the pores as the mat has chemically reacted with the varnish I used on the bench and the 2 are stuck together. Doh! I'll replace it one day.
Re the DC DC buck cct, I've tested the other working example on a 12V filament bulb and the start up is instantaneous, I hadn't thought of any problems regarding firing up DHT filaments with it.
The query was more about the buck converters effect sonically if any? I didn't know if it was a known no-no.