I've been using the AVAA C20 for the better part of a year. I can say it is quite powerful in its ability to absorb unwanted bass modes and works as well as Tom Martin reports. And because they are active, they are far less obtrusive in the room.
However, in our listening space (which is a multi use area of our home and not a "dedicated' listening room), it took some time to find the best location to accomplish this.
As with any bass trap, passive or active, the AVAA needs to be placed in close proximity to the bass mode(s) you wish to treat. Often, but not always, those modes are in the room tri-corners. If your speakers are located near a corner where a bass mode exists, be careful to keep the AVAA as far from the speaker as possible (I'd suggest at least 2 meters). If too close to the speaker, the AVAA will affect the perceived bass performance of the speaker itself. If you are already using passive bass traps in the corner, I would suggest removing them prior to adding the AVAA.
After a lot of experimentation, I discovered the most significant bass mode was in the right rear corner of the room (as one faces the speakers on the "front" wall). Ultimately, the best performance in our listening area was achieved by stacking 2 AVAA C20's in the right rear corner.
As with other bass traps/absorbers, I have found 2 or more work best but, of course, that comes at a cost in both dollars and space. Fortunately, because these devices are active, they are far more efficient than passive bass traps. In fact, after I added the AVAA C20's I removed all the ACS tube traps I was using previously.
