I agree with going for the QS4s. I wouldn't invest in another center until you can match the center to your mains, otherwise you will suffer the same disappointing effect again. Save up for an Axiom center to match.
Just because I was curious from all I read on these boards, (I often feel strangely compelled to prove odd things for myself when I'm bored) I tried using an old HK bookshelf instead of my VP150 that nicely matches my M60s. Even though I still consider the HK bookshelf a nice speaker in it's price category, it just didn't match the sound of my M60s and my sound got kinda muddied.
That being said, I'd probably leave the center purchase until the very last and if you ever do get additional funds to expand, I'd get a subwoofer before reinvesting in another center. As you said, you can do phantom center for now, but it's hard to invent a phantom sub.
