Bruce, good to hear that you're enjoying your new M80 refurbs. As to the amplifier changes that you're considering, you should expect no "noticeable difference" in the basic amplified sound with any of these receivers. As with all well-designed modern units, when operated within their designed power limits, noise and distortion are inaudibly low and frequency response is audibly flat across the spectrum. The rating on your RX-A1000 is 105 watts/channel and the others vary between 80 watts(Anthem 520)and 125 watts(Denon X4200). This is only about a 2dB difference in maximum output level and it's unlikely to be significant when listening.
One difference that may affect the sound quality that you experience is the difference in room correction processing. Yamaha uses its YPAO, while Anthem uses ARC(Anthem Room Correction)and the X4200 has the highest grade Audyssey MultEQ XT32. While reports vary, ARC and MultEQ XT32 are generally viewed as superior. So, it's this factor which may be significant, rather than any inherent audible differences in the amplifiers in these receivers.
