The answer is completely dependant on whether 2-channel performance will be an important factor for you. If you are used to good 2-channel performance and want to maintain two channel but also have surround for movies, then keep your preamp and power amp and add a pre/pro and 3-channel amp. If you are more flexible and less demanding for 2-channel, get a pre/pro and amp(s) or a receiver. They are cheap, easily replaceable and not very good for 2-channel performance.
I have owned some of the nicest pre/pros and multi-channel amps and even a couple receiver/processors. None, and I repeat, none are very good with 2-channel performance if you have high demands. All, and I repeat all, were perfectly acceptable to my wifes standards for music. But she was able to tell the differences immediately between doing it the best way for 2-channel and just having 2-channel available.
I have owned some of the nicest pre/pros and multi-channel amps and even a couple receiver/processors. None, and I repeat, none are very good with 2-channel performance if you have high demands. All, and I repeat all, were perfectly acceptable to my wifes standards for music. But she was able to tell the differences immediately between doing it the best way for 2-channel and just having 2-channel available.