Franklin,
I have not heard the Marantz's, but I own a Panasonic SA-XR45 and it is VERY GOOD for the price! I picked mine up here used for $275 and it is a great bargain at that price. It is so good that I would bet 95% of the general public would conclude that it is the best they've ever heard. Of course, that's not saying much because 95% of the general public are "Audio Morons"!! But seriously, it is good. It is smooth sounding and has a lot of unique features like "party mode" for bi-amping a pair of speakers for double the power. It also is an ALL DIGITAL amp and if you use a digital source and use coax or optical input, it keeps the signal in the digital domain all the way through to the speaker outputs.
I have no idea if the Marantz's are digital amp technology, but if they are not, I would choose tha Panasonic.
BTW, My primary 2-channel setups are tube amp based and rather hi-end. The Panasonic is no match for my big rigs, but unlike many HT receivers I've owned, it is listenable, smooth and non-grainy. I could easily use it as my primary amp if necessary.
Also, Boulder Cable is now performing modifications on the Panasonic that take it to a whole new level and many who have heard it are considering selling off their expensive 2-channel setups to go with the mod'd Panasonic. Check this out at www.Audiocircle.com if you are interested.
Lastly, the Panasonic SA-XR45 is discontinued and out of stock nearly everywhere and is in high demand. People are bidding up the price of used ones too. Check out the "want to buy" ads here on A-goN.
The replacement XR50 will be out shortly, so you might want to wait.
Enjoy,
TIC