It may not have been the actual amp power that was the issue/difference, either. I've used both avrs with external amps and not. Somewhat depends on the speaker's sensitivity as well as spl levels required. Don't know if another Yamaha will be much different in any case. They have pre-outs so I'd try the new avr alone then decide whether or not I needed power amps (the processor won't have any issue integrating the external amp). A few watts difference isn't much in the range of avrs, usually less than 1-2 dB difference (keep in mind it takes a doubling of power to gain 3dB spl).
ps If you want a even more impressive change than differences in avr amp power, try a better sub.