I don't think there can be an absolute answer to the Amp vs speaker upgrade debate. I think it is probably on a almost case by case basis.

The reason I say that is I look at my old pair of Bose 201 Series II speakers which were admittedly adequate when I had used them with a variety of amps such as a Panasonic HE-70, Pioneer VSX 815 and 516. I am not saying these are top of the line units but the 815 was hghly rated by S&V so I would not consider it a slouch as receiver's go. However, recently I went ahead and hooked the 201's up to my Gladding vintage amp which is 30 years old and only about 25 wpc. The result is the 201's sound nicer than they ever have before. Would you say that the Gladding reciever was an "upgrade" from the prior newer-more powerful wpc units? I don't think so, it just is a case where the one receiver just mates well to the speakers. I am sure others can site other examples similar to this one.

IMO it is not where you start amp or speaker but where you finish-i.e. did you find the best receiver/speaker marriage that produces the best sound?