I would have to say quality and accuracy of sound. A sub has to be almost transparent and not really noticable, but definately there when a low passage is needed.
I am not familiar with the av123 brand, but if they are looking for ideas, I say stear toward accuracy. Yes the "wow" factor is what all of us enjoy, but an "in your face" sub gets tiring very soon.
I remember the first time I heard a sub, oh back around 1974 or 5 and I always remember saying that that is the way to go. So after 20 some odd years later, I retired my Advent utilities and finally got a sub.
I don't know if I am just getting old, but I actually prefer very little bass during normal operation, but when it's needed, oh boy, does she make the house shake.
Colin