Heh, just read your posts. I did some additional research on the STP-SE. It’s amazing that people will have many opinions on "what is the best preamp". Most people don’t understand that system synergy is critical and a particular preamp may shine in one system and sound poor in another.
Repeated comments on the STP-SE say that it is very transparent and act much like a passive pre. However, it also has the beefy power supply and gain circuit that is able to "push" difficult bass/midbass waveforms into the amp. This means you have a highly transparent and fast/dynamic preamp. The key word here is "transparent". The Krell, on the other hand, has 2 Class A circuits (input and output) that the audio signal will travel through. The Class A circuit will definitely color the sound somewhat, but the Krell implementation is still very fast and dynamic.
Your Marantz is going to have a warm/rich sound. The Emotiva Gen 3 uses the XPR amp board which is on the smooth/refined side of sonic signature (though I don’t know how it will sound with the Gen 3 switching power supply). But it’s nowhere near the smooth/slow/lush of a tube amp. (still definitely strong solid state).
The STP-SE will probably reveal the full resolution of your Marantz. On the other hand, the Krell will have a little color to the sound (could be translated into fully midrange body and bass/midbass). Since the Marantz warmth may be similar to a Class A circuit, it may be pushing too many Class A steps in the audio path for my own personal preference. However, I believe the Krell is going to be significantly better than the Emotiva in resolution, dynamics, excitement and clarity anyways.
I guess it depends on what you want, and it may be impossible to know without trying. Based on what I have read, I would say that the STP-SE definitely has an edge over the Krell on speed and transparency. But like you said, it is $900 more. Wyred 4 Sound does have a 14-day trial program. The end-cost is shipping plus 5% ($100 in the STP-SE) if you return it without purchasing.