I had the same problem that you did. I run Infinity IRS V on the front, IRS betas for surrounds and an IRS Gamma on the center. I use different amps for the front and back. The difference is sound field is VERY negligible. I could make the sound totally seamless either by upgrading all my speakers to IRS V and better amps, which isn't something I can afford, or I can downgrade my mains to equal the quality of my surrounds, which is also something I am not willing to do.
Any system that plays a large percentage of music would do well to upgrade the mains at the expense of the other channels.
Any system that plays a large percentage of music would do well to upgrade the mains at the expense of the other channels.