I would think that if this device is the "real deal" ie not some signal processor that most people like at first, then get tired of; which I am concluding from all the positive posts that it's not; then if one were to hook two in series the sound shouldn't change very much, if at all. The reason is if it restores lost info then the second one in series would have no more lost info to restore, therefore not changing the sound at all. Now that's just theory and I figure some small changes, but not like twice as large an effect. If the second one makes a big difference to the sound then it probably is just another spatial effect machine.
So if possible, can someone try two in a row to test this?