Complex sentence: A big sheet of steel hanging in the studio acted as an unconventional echo plate, and of course they indulged in all that slowing-down and speeding up of tapes and looping that has been a trademark of Hopper's music since the early days.
Split into simpler sentences: A big sheet of steel hanging in the studio acting as an unconventional echo plate. They experimented with tape loops, a trademark of Hopper's music since the early days.