upgrading... which amp?

The 4410a's are rated at 125 watts at 8 ohms. And the 4408's are rated at 100 watts at 8 ohms. Is one 5 channel amp a good idea? Or should I consider a 3 channel for the fronts and a 2 channel for the rears? Is adjusting the output level to the rears with a pre enough? How many watts are too much for each channel?
Thanks

First, exactly what do you mean by the Sony becoming "tired" lately? Usually, a receiver either works or it doesn't. Have you checked to make sure the speaker cable connections are snug?

Aside from that, I wouldn't worry about having too many watts of power, as what's important is not having too LITTLE power, which could result in the amps clipping and harming the speakers. If you do want to try a new receiver, you might try something like an Onkyo TX-NR929 and see how you like it. The 929 likely has a more solid amp section than an older ES series model.

What is your budget for a new amp/s? Real heavy weight class A amps will provide punch but is expensive. The Class D amps which are gaining in popularity and use are considerately less money, but may not please you sonically. You have to figure out a way to audition these amps.