I didn't realise I never replied on here! Here's a C&P of what I said somewhere else...
First of all, I'd like to say these are both excellent sounding receivers as far as music is concerned. I think most people would be more than happy with these, except maybe the hardest audiophiles.
Neither of these amps are run in, they've been turned on for an hour or so to warm up, so their sound may well change a little as they do.
Both amps were being fed by a Naim CD5i via Chord Chameleon Silver Plus interconnect, and the speakers were being fed with Chord Carnival Silver Plus.
First of all I ran the Marantz with the Monitor Audio RX6's, as this is the sort of speaker that people will be using with this sort of amp. Initially, using Neil Finn's Try Whistling This CD, which tends to sound quite muddled on cheap systems, things were sounding a little overblown and bassy, which i sort of expected from the MA's, but I shifted them out another foot from the wall which seemed to minimise the problem. I think music with heavy bass on this amp might need more careful speaker partnering, so slightly leaner sounding speakers would match up a little better.
Switching to Rage Against The Machine's 'Killing In The Name', something wasn't quite sitting right with me, but I think that's because I'm used to hearing these tracks on Bryston/KEF Reference. I settled into it, and it sounded fine, and with plenty going on in the track, I thought it was a good time to switch to the LX83. Immediately, the Pioneer was giving me more of what I was used to with these albums - more forthright, and the slap bass of Killing In The Name stood out a little more due to better defined leading edges of instruments. Bass seemed better controlled, but it was obvious that the Pioneer is a leaner sounding amplifier. Going back to the SR7005, it was just a little soft for my liking.
Changing to the ATC SCM19's was almost the opposite. The ATC's are a leaner sounding speaker due to having a far flatter response than the MA's, and the sealed cabinets don't exaggerate bass. Immediately, the SR7005 sounded much better, far more like a real hi-fi system. Bass was far better controlled, treble was tidied up, and midrange was nice and clear. Obviously this is mostly down to the ATC's, but the ATC's let you hear the amp pretty much as is. Changing to the Pioneer, everything was a little too lean. Those familiar with Naim's old IBL/SBL speakers will know what I'm talking about. Treble and mid were fine, and again, the Pioneer pushed things forward a little more - the Pioneer seems to be more of a "here, have that" type of amp, whereas the Marantz is a little more laid back. Bass was nice and tight, but was lacking warmth and depth. The SCM19's sounded nicely rounded with the SR7005, and I think people would choose this combination rather than the SCLX83/SCM19 combo as it just sounds 'nicer'.
Anyone who says that ATC's are easy to drive, this is just another example of how they're not. The RX6's I was playing at -15dB, and the SCM19's at -12dB.
One thing to mention is that our room generally tends to sound a little bass light, as the floors are suspended (bouncy) and all walls are like partition walls due to the building being the timber framed Victorian style. I know that a better bass response is achieved in a normal house, and the volumes I was playing at aren't needed.
So there's a quick comparison for you.