Center channel matched with Fronts??

All speakers should match. Identical if possible.

For 5.1 try turning your center on end (the tall way). Physically set the LCR speakers all the same distance from your chair say 12' radius for example. Make sure to change your LCR distance settings to zero in the processor if you try this.

What is your processor amp/receiver combo?

Adding warm speakers to the rear or center will not fix your problem. It's like having 4 different tires on your car. Yeah it goes down the road, but it will never handle very well. The whole idea about surround sound is to create a seamless presentation. No one speaker should draw attention to it's self. Each speaker should image to the next just like a stereo pair. Forget all the marketing BS about special center and surround speakers. All the speakers should be the same (5 2way bookshelf speakers for example). Next best is to have at least the same drivers.

Surround content is approximately 50% center, 40% R&L, 10% Surround. A small center channel speakers will compromise any system. A common mistake. Sideways center channels with MTM driver configuration also have vertical dispersion, yet anther problem. Don't believe me? Turn your LR speakers on their sides and tell me how great they sound. An RS6 is what you really need, but probably not practical standing in front of your TV. If your speaker is too bright, I would start over with different speakers or try changing your electronics. Most receivers are bright and glassy. Better gear help might solve your problem. Sorry, I kind of rambled there...