I love those speakers. Have had them in my home a few different times over the last MANY MANY years, lol. The brand new Vandersteen Sub 3' system would work great. I know they are very very fast subs (I own Quatro's and know how fast they are). Their amp mimics the amp in the main system, which is one of, if not the only one that does this. It's a simple, but great idea. It also includes an EQ on each sub so you can dial in the bass so you can easily smooth it out in any room.
I have a very difficult room and it works GREAT. As we all know, it's hard to get fast sub bass that will integrate well with the ET's. You have some good options, but make sure to set them up properly or you will never be happy. The ET's have a special magic and you don't want to ruin them with subterranean bass that isn't fast enough to keep up with the panels and won't integrate with the ET's own bass driver. That's one reason I love the amp that mimics your's. JMHO
I have a very difficult room and it works GREAT. As we all know, it's hard to get fast sub bass that will integrate well with the ET's. You have some good options, but make sure to set them up properly or you will never be happy. The ET's have a special magic and you don't want to ruin them with subterranean bass that isn't fast enough to keep up with the panels and won't integrate with the ET's own bass driver. That's one reason I love the amp that mimics your's. JMHO