Changing Preamp tubes on a Cronus Magnum?

In an earlier thread, I mentioned getting my Cronus Magnum on Friday. The amp was well broken in when I got it and I have put at least 24 hours on it since receiving it Friday morning.

I want to ripen the midbass/lower midrange a bit and from what I understand, it's the center 12au7 tube that makes the most difference. What would be a quiet tube that will ripen the sound a bit? Thank you.
Call Rogue Audio and speak with either Mark or Nick. Nobody knows tube upgrades for their amps better than they do, and they will happily help you out.
For me, this really worked quite well ..... in the center small tube slot, an RCA NOS 5814 replaced the original Electro-Harmonix 12AU7 gain stage tube.
I have a Cronus Magnum and experimented with a bunch of different NOS tubes, I finally settled on the following :

Preamp - Cifte 12AU7
Section Drivers - Mullard CV4003
Phase Splitters - Raytheon 5751 w/Windmill getter

This setup gave me the best all around sound (nice midrange with some sparkle on top).

I tried a Mullard 4003 in the preamp position, but liked the Cifte much better. I also tried Telefunken 12AX7 smooth plates in the splitter positions, but liked the Rayhteon's better.
I have the Cifte in the pre-amp position on my RCM as well. Very nice tube, 3D soundstage is correct. The Cifte is flanked by two La Radio Technique Green Box 12au7's. I have 2 Mullard 12ax7's on the way to replace the new production Tung Sol's in there now. Anxious to hear them.

Soon I'll be getting a nice Mullard 12au7 to switch out with the Cifte to see which I like better.

Got everything from Kevin at Upscale, very happy with his selection and customer service.
@ Bajaed

I actually like the Cifte better than the Mullard in the preamp position for the 3D soundstage. I'd be interested in hear your opinion once you've tried the Mullard.

I've gotten most of my tubes through Upscale, Kevin was very helpful.
Hi stereo5,

I agree with adam18 regarding a n.o.s. 5814. On my side I had a GE 5-stars, triple mica from de 1960's. Make sure that both triodes of the tube you'll use are perfectly balanced.

In the time I had my Cronus, I exchange some email with Rogue's owner Mark O'Brien and he always had good suggestions for tubes in this amplifier.

I bought a cryod Mullard 12au7 nos tube from Kevin at Upscale Audio. I noticed the difference as soon as I inserted the tube as it was quite dramatic. I received it earlier today. The Mullard seems to have done the trick. There is now the desired richness in the midrange and I am quite pleased with the results. Thank you all for the help.