I agree. Not too long ago I owned a Marantz avr, then a Denon avr back to back. Needless to say, I did NOT really hear that much, if any, of a difference between the two. I will say, however, that the Cambridge SR20 stereo receiver I owned was much more polite and laid-back if you will. Perhaps, a bit more rounded and smooth. Hence, I would not call it warm, but rather more easy-going.
There are differences sometimes between various brands. But, specifically between Denon/Marantz not so much imho. I prefer Marantz myelf, but usually end up w/a Denon b/c Denon is more affordable here in the US. It is more I just like the look of the Marantz.
Cheers,
Phil