First Watt SIT-3 preamp pairing advice

My advice would be that if you are looking to land on the cooler side of neutral rather than full-on classic tube bloom, the LTA would be the better choice.

@zm I have read this so many times from so many people that I take it as fact. 

The LTA is great, but falls "on the cooler side of neutral" as you state.  If you're looking for warmth, it's not the place to go.  In that case the Don Sach's is a good candidate.  I have owned and loved one.  Note that its sound is heavily determined by the the output capacitors and tubes used.  You can read on my system page for some information in those regards if interested.  My speakers aren't naturally warm (accuton drivers), so adding a cool-leaning preamp is a no-go for me. 

I suspect the Supratek sounds similar to the DS, but gives you a higher gain option, and has an external power supply which is a + in my book.

I have to put another vote in for the Supratek.  I have the Cortese LCR.  I drive Rethm Saadhana (100 dB sensitivity).  Cant say enough about the sweet sound.  I prefer analog, but do listen to a lot of Qobuz/Tidal due to me limited vinyl collection.  My ExoGal Comet DAC does help keep streaming more analog sounding also.
And value of Supratek is outstanding!

Thanks @cal3713 for your impressions! Very helpful. And i tend to agree that anyone that spends time on creating a highly engineered outboard power supply likely has a good product. I am in the camp that a good portion of the sound (quality) of a piece of audio equipment comes from the power supply. 

The MZ2 has more air and transparency. I would agree with zm's characterization of the MZ2, but it still has that tube liquidity. I do think the other 2 choices are good pieces, and I still haven't sold my Supratek Sauvignon . I've been thinking about building or buying a Don Sach's phono stage.  My MZ2 has remote volume control and an external power supply, they are both options and it is considered a Stereophile A rated headphone amp. With  both options it costs a little over 2K. The pictures of the MZ2 don't give the fit and finish of the unit justice. I believe the case work is done by Fern & Roby. It is very well built.
I was going to purchase the Pass HPA-1 but it doesn't have a remote control. As I'm writing this response, I'm listening to Ronnie Earl and I feel like I'm in the studio/hall with them.

Hi Andrew, 
I have a SIT3 currently paired with a "The Truth" passive preamp with photocell attenuation.. My speakers are Cube Nenuphars (92db, 6ohms). I have no gain issues at all playing digital but analog is my primary source. My analog front end has plenty enough volume with only one or two exceptions of records that are relatively quiet pressings. The recent pressing of "Abby Road" being one. I've been very happy with "The Truth". It's about as pure as it gets. But I am feeling the itch to explore other possibilities. I had a MZ2 for a short time once when I was using a 2A3 SET amp. The ZOTL did not pair well with the SET. It made beautiful pure tones but the sound stage and 3d dimensionality of the 2A3 amp went flat. Still, it was an amazing headphone amp and I miss it. I'm certain it would have been completely compatible with the SIT3. I am seriously considering the MZ3. I would like to buy another MZ2 and save the cost difference for other things. But the word is that the sonic delta between the MZ2 and MZ3 is more than worth the additional cost. The SIT3 and Nenuphar deserve a preamp of equal endgame caliber. LTA says the MZ3 is the full realization of the potential of the ZOTL circuit.  I am certain there are other vastly more expensive preamps out there that would be appropriate for the SIT3  but  I'm not sure *any* more costly pre would actually be better in my system. At least I'm willing to see if this is true.
So yes, I intend to purchase an MZ3.