It also depends somewhat upon bass management and intent regarding multichannel audio. That is, the QS8's have bigger drivers and a bigger box, so they just go lower, flatter than the QS4's. If your receiver has very sophisticated bass management AND/OR you don't care much about the prospect of multi-channel audio, you can probably do just fine with the QS4's. The design principle holds true, and I think the QS4's are quite a bargain.
But I wanted to avoid any potential for buyer's remorse, so I bought the QS8's
Likewise, Ray will likely chime in that the VP100 is a stellar center channel and only suffers in comparison to the VP150. Really, Axiom makes fine products and you don't necessarily have to buy the "ultimate" models to benefit from their quality in design and production, especially if you have the discipline to stick to a budget or don't want to "wait" to enjoy great audio.
