Old quote, but I forgot to address it. There are a three main reasons why I think the xr45/XR10 sound better than the xr57, even though I have not heard it:
1. Power supply. Like I said in my earlier post, the TI PWM chips used in all of Panasonic's receivers have a modulation scheme that relies more on the quality of the power supply. The less robust of a power supply means the chip has a less accurate timing, creating a reduction in sound accuracy. Out of the entire Panasonic lineup, the xr10/45 have the most robust power supply to date.
2. Speaker protection relays. There are speaker protection relays in the newer Pansonic receivers. This is a safety feature in case a speaker shorts out. However, some feel these relays affect sound quality in a negative way. The xr45/25/10 do not have these speaker relays.
3. This applies to the xr10 only. The XR10 uses discrete components for it's amp stage on all channels (Three tas5012's with discrete parts). The xr70 has discrete amplification, but only on the front left and front right channels. The rest of the Panasonic lineup uses the cost effective tas5076/5036 pwm processor with three or four tas5182 amp stage controllers.
In addition, since the XR45, all Panasonic class d receivers have a *very* similar internal design. The xr10 is the only different design/part layout. There are many internal pictures posted on Panasonic's lineup; do a search and you will see just how similar they are. Many of the parts on the xr45 are still used in the xr57 (for instance the tas5182 amp stage controller is used in both the xr45 and xr57). With similar parts and very similar design, it's safe to say Panasonic's lineup as a whole will sound similar to one another.
Since all things parts/design wise are pretty much equal across the lineup, I think the lack of the relays and the addition of a more robust power supply make the xr10/x45 better. However, like I said before, the "dual-amp" feature in the newer models allows for more flexibility in the type of speakers used in stereo mode. I think this speaker flexibility for stereo listening makes the newer models more attractive than trying to find an old xr45/xr10. In terms of sound quality, after listening to an XR45, XR10, and XR70 in my setup, I liked the XR10 the most. What I'm waiting for is to see if Panasonic makes any design changes to the XR700.