I am not a big fan of the LSIs (love the tweeter, hate the midrange), but since I haven't heard the ERTs, I can't really say. The 15s are already 4 Ohm so if the PM8004 is handling them fine (if you've tried them on it), then the ERTs should also be fine. Even though Marantz lists all their amps at 6 Ohms minimum, I've run 4 ohm speakers on my PM7200 before; it doesn't go really loud, but it gets loud enough for typical listening.