That doesn't seem universal either.
Let's take an extreme example: a L/R 3ft apart. At any real distance they are going to basically become a point source with the center about the middle of the two.
Adding a true center channel (a small one so it will fit), I am convinced, will provide no benefit unless the listener is near-field (1-3ft away).
My setup has the L and R speakers maybe 8' apart (offset toward he right room wall) in a room about 14' long. In the corner farthest away (left rear corner where my massage chair is), the sound remains better.
I'm not disputing other people having other results; but the on/off axis rule, if it exists, is far from universal.
I have to agree with krzywica. I tried phantom in one particular setup and found that it only worked well if you were in the sweet spot. As soon as you weren't, things became odd with voices and I found it very distracting. It does depend on the speakers in question and their off axis performance as well as the room and where you sit. In most cases, I would still say that a center will be a benefit if you have the ability to position things relatively "correctly" (good positioning, good room, proper seating position, etc...). I agree that with mains at 3ft apart, a center is useless though.
For a center, it depends on the versions you have. If you have the more recent Monitor Titans/Atoms, then you would want a CC-190 or higher to match them. If you have the older versions, a CC-170 or -270 used should be fairly easy to find.
For an inexpensive amp, check
www.partsexpress.com You can get an Audiosource Amp100 for $150, at which point you might simply consider getting an active sub from them for around the same price - the Sub100 or Sub120. You can find the Amp100 used for pretty cheap though; I picked one up nearly brand new for $50.
