Someone on Pinkfish has suggested that the filler in the Entreq boxes looks like granulated copper slag which is used for shot-blasting metal (prices from $100 per metric tonne). Google it and and you will find plenty of photos. It certainly does look like that material.
IMac macOS 10.15.3 (no link to Devialet ) / MacBook Pro Retina OS X 10.14.4 / Linn LP12 / Devialet 200 / Wilson Benesch Discovery.
Qobuz Desktop Latest Version / Audirvana 3.2.18 / Audirvana Remote / iTunes 12.9 / AIR 3.0.4 / Wi-Fi / FW 8.1.0 / SAM 50%
Cambridge, UK (Updated 27th February, 2020)