The guy is trying to inform you but apparently you are very closed minded. You really did get that system for a pretty good deal but like everyone has already said, it isn't worth spending the extra money on a sub. You WILL NOT get better sound out of your system with a 100 watt sub added to the mix unless you associate more bass to "better sound". A good pair of bookshelves will give you better sound alone than a sub, acoustimass, and those cubes all together. He is also right about another thing, those speakers do not have crossover's built into them, the crossover is in the Acoustimass module. That is not a good thing because if you try to hook them directly up to your reciever, they could very easily be damaged as they are only able to respond down to 200Hz or so.
Your comments about him just wanting to hear himself are absurd. You should be happy when you ask somebody a question and they tell you more information than you anticipated. Learning can be very rewarding but it seems that you go out of your way to remain uneducated.