I’m not sure if the quest to “dramatically improving the sound” still includes the idea of using two sets of front speakers. But assuming a single set of speakers, if a new amplifier “dramatically improves the sound,” then something was horribly wrong with the first one.
I’d say unless you’re hearing distortion from your speakers during demanding movie passages, then you don’t need a new amp. The “rule of thumb” for adding an outboard amplifier has typically been that you need to double the power to make it worth your while – e.g. a 200 watt amp if the receiver is 100 watts. This will get you an additional 2-3 dB of volume from the speakers.
Regards,
Wayne A. Pflughaupt