Welcome to the family Doug!
Here is my opinion on some of the questions you posed above:
Quote:1) Even though the Rachel and UF02.1 amps can be used without a preamp ahead of them, what exactly are the pros and cons (if any) to utilizing a preamp ? And will the preamp likely offer a little more overall SPL "headspace" by providing a bit higher mV signal to the amp(s) ?
Most sources can put out enough voltage to drive your amps to the max clean output so adding a preamp usually doesn't add any more actual SPL. But a good preamp can make it
sound louder, or just better (how ever you define it) by tweaking the sound.
Quote:2) Are there any Decware SET amp users who utilize a non-Decware preamp in there signal chain ?
I used a Woo Audio WA22 as a preamp (and headphone amp) for my UFOs (since sold) and will for my UFO25s when they are delivered. I think it is awesome. I learned very early on that I wanted a tube preamp in my system, which for many years was an Audio Research preamp and Pass SEM (single end MOSFET) power amps.
Quote:I am wondering and curious about utilizing a "non-tube" (SS?) preamp. Feeling like the value (magic?) of tube amplification happens in the SET amplifiers themselves
There is a lot to unpack there. Firstly, I'd say that "tube magic" is in the ear of the beholder. SET amps certainly have one kind of tube magic but tube preamps also have magic, although of a different and perhaps more subtle sort. I wouldn't look at the preamp question as tube vs. solid state so much as good vs. not as good. Steve Guttenberg (the guy who made Decware go viral with his rave youtube review) used a solid state preamp. There are a lot of great sounding components out there, way more than anyone has a chance to listen to or evaluate.
Quote:I really like the idea of having in-depth numerical testing by electrical engineers
Sure. My thing (as I have posted many times) is I'm a physicist by training and trade but just as no one has ever bought a Stradivarius or a Chateau Margaux or a Rembrandt based on measurements I'm not going to buy audio gear
based on measurements either. I know what I like and I trust my ears.