I went through this scenario trying to plan an amp integration in the future HT. Socketman helped me out with some good links. Here was my takeaway:

If the preamp is unable to drive the input sensitivity of the amplifier to max, the amp will not be able to use all of its power to drive the load.

From my understanding, if the preamp and amp are not closely matched noise issues can occur. If the amp has very high gain the output noise floor may be increased at listening volumes. I'm not sure if the preamp output is too low the amp may clip to drive the speakers to desired volumes. I just think it wont reach desired loudness.