What Preamp Tube Sounds Best 

Yes, 12AU7 works best for me. It goes somehow into Fender territory, taming distorsion, so can get more cleans in my Micro Terror, (not Dark)

I swapped several tubes through my Micro Dark. I tried a GENELEX Gold Lion 12AX7 Gold Pin, JJ 5751 Gold Pin, JJ 12AY7 Gold Pin, JJ 12AU7 Gold Pin and even an old GE NOS 12ax7 I pulled out of an old Hamm Radio. I personally think the JJ 12AU7 had the best overall sound, it took my pedals really well too. The GE NOS sounded fantastic, was my second favorite. Great when you crank the gain up. I guess it depends on what you are looking for? The 12AY7 had the least amount of output, the 5751 was okay but had a very harsh treble response. The Gold Lion was the loudest, I thought sounded great but REALLY was hard to control the gain. It would start breaking up quicker than any of the other tubes. I stuck with the 12AU7, I use the MD for worship gigs and it takes my pedals the best and sounds like I'm playing an actual tube amp.

Not sure if it'd be the same for the micro dark, but changing in a tung sol 12ax7 into my dark terror was a pretty drastic improvement over the stock JJ

Just got mine with a Ruby. I can get cleans, real cleans with the volume dimed. My max gain is nowhere near the roar I’ve heard. Not sure what I think. The cleans are really nice and I have lots of pedals to boost the front end. The max gain is a great sound as well. Going to go play this thru a 4x12 right now to see what it can do.