Why are my SF Venere S Speakers not sounding as warm?

The Marantz 6012 has no balanced circuitry, so there is no reason to connect to the amp's XLR inputs.  I think you just need the best RCA interconnects you can get. 

Wireworld Silver Eclipse 6, 7, or 8 seems to be a very nice sounding interconnect that you can often find used here on Agon.  For more cable suggestions, you might try asking in the Audiogon cables forum.

Your Marantz SR6012 receiver has no XLR pre-outs. You can use a pair of RCA cables to connect to your Krell stereo power amp. But still for your stereo music playbacks you will need to get a separate dedicated stereo preamp with HT bypass. And connect your front end source components(s) for your stereo music directly to a stereo preamp. So when listening to stereo music materials you don't even have to turn on your Marantz SR6012 receiver at all. 
The Marantz receiver's preamp section isn't good at all for stereo music playbacks. 

@tice34  
  
For Audyssey, use all 8 microphone positions. The first one should be dead centered where you sit, all other 7 should be where your head could be if you are slouching, leaning forwards, they should not be more than 3ft from the 1st position. Also, try to avoid placing it right up against any headrests.
 
Have Dynamic Volume (a dynamic compressor) off.
 
Have Dynamic EQ on. However, it’s default is for the typical mastering of movies, change the Reference Offset based on the type of content you listen to. 5dB for classical, 10dB for tv/pop/hip-hop, 15dB for heavily dynamically compressed music (a lot of metal/rock). I watch a lot of movies and tv and play a lot of hip-hop, so I go in the middle and choose 5dB, as the movie default results in way too much bass for music. 
 
What Dynamic EQ does is use the Equal Loudness Contours to adjust the frequency response based on playback levels, so that it always sounds neutral. However, you have probably noticed that if you play a movie on your system and then switch a pop/hip-hop song, you lower be volume, so that throws the Dynamic EQ off because they are mastered to different levels, so that’s why the offset is available. 
 
If you feel you can’t get Dynamic EQ to work with your tastes, then it off. 
 
If you feel you can’t get Audyssey right, use the L/R bypass, so it only corrects the subwoofer.

Mzk,
Wow, thank you very much!  I will re run Audyssey when the amp is hooked into the system and follow these suggestions!

"What do you all think of the pre amps coming from
http://www.tubes4hifi.com/custom.htm
and or Don Sachs?"

Don Sachs makes an excellent preamp but you may need to wait since there's usually a wait list. You'll not be disappointed.