Here's one that'll make you smile :) (And yes, this is a serious suggestion...) replace your TV with a front projector - and then use a retractable screen. This will eliminate a large sonically reflective surface right between the main speakers, becaues the screen will be retracted when you're listening to 2-channel. It has a surprisingly audible impact on 2-channel imaging. I learned this by accident, because I upgraded to such a setup purely due to space concerns when I wanted a big screen, but until I find enough money to build a dedicated theatre room (as opposed to a combined theatre and concert room), I'm not going back!
Also, I've gone down the same path as you have - my speakers aren't identical but they're pretty close. However, I discovered that even adding some very different surround speakers made less difference than I would have expected. In my case, I upgraded my surrounds from Triad Silvers (dipole in-wall cone speakers) to MartinLogan Scenarios (bipole floorstanding electrostatics), and I found that both worked equally well with the main MartinLogan panels up front. Their signatures are different, but this is only perceptible when playing 5-channel audio. I don't perceive the difference AT ALL when watching movies, except that the Scenarios obviously have more bass punch. There is a difference in 5-ch music, of which I have little so far.
I can also report that my amplification is quite different front to back, and this too is not an issue. I have vertically biamped my ML Odysseys with a Classe CA200 for the cones and a Counterpoint SA220 for the panels. (This took a little tweaking!) That's the equivlaent of 800wpc@4 ohms with mixed tubes and transistors for the fronts. The surrounds make do with just a single 3-channel B&K ST3140, at 175 wpc@ 4 ohms, all transistor. This is a pretty big difference, and the only one that can be readily identified is that there is some timbre change from the mains to the center and back. I don't notice the difference to the surrounds very much. I will have to upgrade the center amplifier sometime, although I'm having a hard time imagining precisely what I would put there...
Also, I've gone down the same path as you have - my speakers aren't identical but they're pretty close. However, I discovered that even adding some very different surround speakers made less difference than I would have expected. In my case, I upgraded my surrounds from Triad Silvers (dipole in-wall cone speakers) to MartinLogan Scenarios (bipole floorstanding electrostatics), and I found that both worked equally well with the main MartinLogan panels up front. Their signatures are different, but this is only perceptible when playing 5-channel audio. I don't perceive the difference AT ALL when watching movies, except that the Scenarios obviously have more bass punch. There is a difference in 5-ch music, of which I have little so far.
I can also report that my amplification is quite different front to back, and this too is not an issue. I have vertically biamped my ML Odysseys with a Classe CA200 for the cones and a Counterpoint SA220 for the panels. (This took a little tweaking!) That's the equivlaent of 800wpc@4 ohms with mixed tubes and transistors for the fronts. The surrounds make do with just a single 3-channel B&K ST3140, at 175 wpc@ 4 ohms, all transistor. This is a pretty big difference, and the only one that can be readily identified is that there is some timbre change from the mains to the center and back. I don't notice the difference to the surrounds very much. I will have to upgrade the center amplifier sometime, although I'm having a hard time imagining precisely what I would put there...