Today afternoon I was spinning "Get On Board", Taj Mahal and Ry Cooder. Can't get enough of this album, but "Packing Up Ready To Go"... that song is something else. They sure found their sweet groove. Should you not feel the presence of that bass drum in your room, change your system.
Oh, yes: recording is great, mastering is very, very good - and appropriate. It is all captured and preserved, from sweet rawness that usually get conjured during casual jam in a forgotten roadhouse, to the reverberation of the room they recorded it in. Absolute treat for music lover and audiophile.
My 3 most important things - for my system, not only for an amp:
1. True to the recording - reproduce what is recorded, no more, no less. If there are "spatial cues", I want to "see" the room.
2. As close as possible to the real voice(s) and instrument(s) - reference is the live event and, in my case, years of being musician
3. Goosebumps. A lump in my throat. On some recordings happens. On most - it does not. I need to be taken, transported to the event, even for just a moment. System is the vehicle. Music is the fuel. Rare are the pieces do have such a beauty that can transcend poor reproduction. Conversely, if music is crap, no system can make you suddenly hear The Masterpiece.
In too many years I'd care to mention I had many different systems. In early eighties, I had one ghetto blaster in my apartment. I even tried D class Bel Canto with Dynaudio speakers. Now, I am on my second Audiomat Prelude Reference. I sold the first one, to "try something different"... I also have Audiomat Phono 1.6 - which would classify as SS. My MD806t uses two 6992. Speakers are Capriccio Continuo Auralea 309, very good match with the amp, and the room.
I prefer good sound, and don't care is it tube, SS, D class... nor I care about features. Less is usually more: I prefer integrated with passive preamp - power amp with input selector and volume control.
I heard two different Zu speakers, one with Decware EL84 and another one with Trafomatic. If I would have your dilemma, I would try to hear those manufacturers. Trafomatic was more to my taste.
Personally, the only integrated amp, besides Audiomat, that I would buy is Ayon.
Also, tube is just a part of the design. I heard EL84 design that was well balanced, with a nice and tight LF output. Also heard 6L6 design with mid-bass hump, and quite flabby down low. Good mids, but nothing else. So, which tube should be followed by which iron, which caps etc. Avoid all of that and go out and listen. Ask Zu people. Visit dealers. We can all give you great insights, but ultimately, those are only our, ultimately subjective observations. Hearing is very subjective, just like any other sense: taste, touch, even vision - just read the studies on crime eyewitness reliability.