ok, many false statements here.. First, you can run all 3 subs, and it MAY be louder. No, it doesnt matter if they are square, round or triangles. Brand doesnt matter either. The problem is, each sub is going to produce one note VERY good, and the others may produce a different note well. This will matter in a SQ (sound quality) setup, and some sound cancellation can be the result. The average person wont notice it. BUT since the Visonics are a low quality sub, you can easily over power, and as a result distort and blow them before the Solo goes.
That said, do a little research, build a box for the Kicker, it will cost you $30 in MDF and a little time. If you buy a prefab box, it may sound like a good deal, but in the end, you can out perform almost any prefab "ported" box with one built to spec by a noticeable margin.