Never Owned a Tube Amp and Want Advice

LOL I should have just asked first before adding my 2 cents!

Ok, All I can do is recommend the tube amps that, "I", personally enjoy. Like everything else this is WAY subjective. And the speakers will show the amps differently as well.
But there is more to those which I like than just the sound. And the tube power differential people have talked about? That is a "real", thing. I can't say that I understand it very well. But as the different manufacturers test, rate and also design and build differently? Your going to see a wide range of differences because of that as well!
 I have one pair of mono-blocks that only rates at 4 watts but pushes as well as any 40-50 watt tube amp that I have tried. "Sonance, Opera 300B cyber". And I will say that most tube amps that I have tried rated at 30-50 Wpc  will push as well "IMHO", as most SS amps rated for 100WPC. "Not-withstanding, Pass, Threshold and a few others"!
 My favorite tube amps overall are the "Manley Labs", ALL of the offerings from Manley sound crazy nice and are also built like a tank! And the Biasing is the most user friendly I have ever seen. Marked out for you on the top of the amplifiers themselves. And I believe they even still give you a free multimeter when you purchase them from the factory which is in Ca..
Something i have noticed though in the way you want to power your speakers for the best sound which I have never heard anyone else say before though. When I purchase/use SS amps? Headroom is a GREAT thing. It helps keep the distortion low. Keeps you away from clipping which can destroy some amps very quickly. And is just a good thing all around it seems. With the tube amps? Not so much! You want the amp max, "Where it will clip", To be near your reference listening level. Why?
 Because with a good tube amp that is where most sound there very best! But a SS amp? For most, 
"Again though",
"Not-withstanding, "Pass, Threshold and a few other SS amps, that work a little bit differently", You want that headroom to stay well away from where they clip. That for the best sound you can get from a SS amp. Because distortion from a SS amp is always just horrible noise. 
 I also have to agree with "WolfGarcia", above. The most bang for the buck", I believe would be from "Jolida", Just, now it is named, "Black Ice"!
Black Ice, also has a certain, one Mr. "Jim Fosgate", in the house now. "
"That fellow currently holds more patents than even I! And I believe he's near or even past the ,(Four-Hundred) mark now"! And those patents are all related to audio. Which now, "Black Ice" basically, has free-access to!
 And as far as tubes? they do all sound a bit different. And different as well, each in different gear! The 300B is my favorite, "Output Stage" valve to use. But they can be a bit pricey, even for new tubes currently made!

My expectation is that a tube amp will improve the imaging, layered sound stage, and sense of aliveness I think I hear when the treble gets better detail without sibilance.

Right. Should be fine. Only slight concern is when you say the McCormack RLD is too polite. Never heard that one myself. Had a DNA1, beautiful sounding amp. Very, very close to the Aronov and Melody tube integrateds. Very close in sound, very close in perceived power. Which there's the 150W SS to 50W tube equivalence thing. But in terms of sound, if you like the DNA and just want a little better imaging, layering and palpable presence, you should be able to find that with tubes. Any of the amps people are recommending should do that easily. 

Improved treble detail without sibilance is a sort of hallmark of tubes vs SS. The better tube amps provide extended treble that can seem downright liquid. The last guy to hear my system was into digital and while he was here kept thinking my system should be more cold and analytical. Only find himself yearning for that nice full round warm sound that was missing when he got home. So there's something about it that wins over even those who aren't naturally inclined. Seeing how this is already the direction you want to go its a safe bet you will find what you are looking for.

I’d skip the tube amp altogether and go for a class A SS integrated paired with a Tubed phono preamp. That is if your into analog/vinyl, if not then never mind....

I have owned Rogue Audio tube amp products for almost 5 years and find they deliver a sound that I prefer.
I started with a Cronus Magnum II - KT120 power tubes delivering 100 WPC. It gave me a sound that I really enjoyed over just about every other amplifier I have heard. Powerful enough to be dynamic and more than room filling with 84db and up speakers. Power Tubes last ~ 2-1/2 years or so. Bias adjustment is easy- remove a plate with two captured thumb screws and flip the 4 switches to display the current reading on the built in meter. adjust the screw with the supplied screwdriver until the meter reads the correct value.
Takes 5 minutes every 6 months or so. Usually they do not need any adjustment, just check the value on the meter.
The sound is highly refined and detailed, more neutral than syrupy, smooth with a very large 3D sound stage and excellent bass, midrange and treble.
The possible downsides are that it is somewhat large and heavy (still fits on the top of a rack or shelf), does give off some heat and can have some local hum near the amp that becomes silent a few feet away (never through the speakers). Downsides are never an issue for me and the sound quality IMHO beats every solid state amp below and significantly above its price.
The new Cronus Magnum III is supposed to be even better.