The ONLY Panasonic receiver with signifigant upgrade in the power supply section was the SA-XR45.
I think the extra components in the xr45 were more for filtering than power, right? The extra components probably give it a little bit more headroom than the xr25, but I'm guessing the components were mainly there to create a better sounding receiver. The XR45 is not the only receiver with these extra parts; the xr10's power supply has the same type of filtering/capacitors found on the xr45. Here's some internal shot comparisons:
XR55 (56k warning):
http://www.audiocircle.com/index.php?topic=21634&start=0&postdays=0&postorder=asc&highlight=XR25 (sorry, best picture I could find):
XR45:
XR10:
Sound and Vision magazine test results for the xr25 also lead me to believe that the extra components in the xr45 are mainly for filtering. They took down articles from 2003, but luckily I saved the page.
Here is the test results of the xr25 (courtesy of S&V):
Top half:
Bottom Half:
82x5@8 ohm, all channels driven. At 1 watt the xr25 still has .13% thd! The owner's manual for the XR45/25 (they use the same manual) states 100 watts@6 ohm, further suggesting that it is for mainly filtering.
It sounds like the 25 is actually better than the 55 as far as digital stereo playback, correct?
Feature-wise, the xr55 is better than the 25. The 55 and 25 will probably sound the very similar, with maybe an edge to the 55 simply because it has updated TI equibit parts. Wayne is right in that there are speaker protection relays in the 55, however it is uknown how much they affect sound quality. The xr45/25 and the XR55 both use 3 tas5182's, they are the digital amplifier power stage controllers. However, the xr25/45 use an older tas5036 chip for PWM processing, while the xr55 uses the newer tas5076 chip for PWM processing. The xr55 can also "Dual Amp" mode. I have no idea if it makes a difference, but here is what the manual says:
Enjoy a higher quality sound during stereo playback by playing
the sound through the amplifiers used for multi-channel sound
sources.
Just a quick note about the xr70. The front channel output stages for the xr70 use discreet components instead of the tas5182. The front channels probably sound better than the xr55 and x25 fron channels because of this. S&V tested the xr70 to only have .03% thd at 1 watt, most likely on the front channel. The xr70 also tested 77x6@8 ohms.
Overall, if you plan using the receiver for stereo mode only, stick with the xr25. If you are looking for more features in the receiver, consider upgrading.
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P.S.
is changing the power cord then a first "easy" step to improved sound with the Panny XR55 ? which power cord would do the most ?
I'm not going to get into the whole power cord debate, but if you are looking for a cheap way to reduce some RFI, buy this:
http://www.radioshack.com/product/index.jsp?productId=2103222&cp=&kw=choke+filter&parentPage=searchPlace close (oh, maybe 6 inches or so) to where the a/c plug attaches to the back of the receiver. The stock xr10 power cord has this ferrite choke on it (omitted on all other models), so it must have helped with something.
