Hey Bilco,
I used to have an SM7B and Great River... and thought I would never ever ever want to part with them.
I have a Digi 003 rack and patch in. Sounds like we're in the same ballpark.
The SM7B (two of them!) and the Great River (stereo pre!) are now gone.
As my access to a variety of pro gear increased, as I gained tracking experience in a variety of situations, I found that my *tastes* changed along with my working style. Luckily the SM7B and Great River are well respected pieces of gear and I was able to recover my investment in them.
I would suggest you do the same with your 4-banger: buy something that is widely respected and will either last you *forever* or be something that you can sell for a fair price if your tastes/needs change.
From your list, that would mean: API 3124+, Daking IV. But given "name recognition", the Daking won't register with clients as much as the API will. I guess this depends on your clients. Mine actually have *never* *ever* asked or cared about preamps. They might prefer the silver mic or the black mic. I make the final decision and once that is determined, they want to get down to singing or playing.
Regarding simplicity, I ended up with a two channel Hardy M1. It doesn't get much simpler than that. Everything I do past basics has been going through those preamps. One knob for gain. Because of this, all of my attention goes into artist/instrument/room/microphone selection/microphone position... which is where it should be.
The API and the Daking would satisfy your simplicity criterion as well. I have rented a 3124+ and a Lunchbox before. No personal experience with the Daking... but if it's at all related to what Ken Scott was doing in the early 70s... I would be happy to get on board with that.
My solution for X based on your criteria: API 3124+. Wow... surprise!
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Long aside:
The Hardy M1 doesn't have the same "gearslutz instant recognition and sellability" as a 3124+ but it does have the feature to buy a 2 channel unit and then fill in the channels as your budget allows. An API lunchbox would be similar. John Hardy was available by telephone for questions and followed up by calling back if I left a message. The pre has been solid on everything ... vocals, drums, piano, etc., etc., etc..
Here's a twist. I actually found that putting the preamp beside the artist (via short, fresh, high quality cables) and then running whatever distance via amplified mic signal to the 003 made a much larger improvement in quality than any futzing around with patching preamps in my "control zone".
Yes, it's a bit different from the Great River (I had them both for a time) but I haven't missed the GR since selling it. I actually found it annoying to have the option to use either the GR or the Hardy, and was breaking the flow when I went to repatch only to find that the net difference between the Hardy/GR was vanishingly small compared to moving/changing microphones, changing guitar picks, asking the musician to change their touch slightly, etc., etc.