Yes IMO there is no need to match up a subwoofer with the rest of the speakers. That being said some subwoofers are definately more accurate than others, and I feel when you spend money on Axiom subwoofers you are paying for the accuracy that they give. However I have heard really postive things about companies such as HSU, SVS, eD and a few others that come in at a much lower price point. So if it were me I would give them a try, if I'm not mistaken most of them also have trial periods in which to test drive them out.
And the same goes for the on-walls compared to the bookshelves. If it's on walls you're after then I say order the on-walls and a sub and see if that combo satisfies your needs. If you feel the mid-range is lacking you can always send them back for the bookshelves and see if they fill that gap better.
Can't wait to see what you decide to do, and how they perform for you. Good luck!
