This is my opinion, no more than that, and less than $0.02.
"Sound good" to me is the sound of the living and breathing singers and musicians: immediate, clear, as natural and emotional as artists made it. After many years as musician and audiophile, I am still in the camp of "less is more". I still think that tubes are better because they sound more natural, immediate, probably because the signal travels through vacuum, as opposed to Germanium, Silica etc. in transistors.
Regarding a subwoofer - to hear 20Hz sine wave one needs the room where the seat is at least 17.2 m away from the source (speaker). Even the 30 Hz wave needs more than 11 m. Our bodies can feel those vibrations passing through, but that is not the sound yet. However, that is entirely different topic. To me, coherence and timing are most important, followed by range. Good SET tube amplifier is already very fast, and speaker which can transfer most of that speed can sound very natural. The snare will have that naturally compressed snap, you will be able to hear buckling of the skin of the kick drum, and cymbals will be immediate without making your ears bleed.
After many different combinations, I have now Cary 300SEI and Reference 3A Veena. Synergy is great, and in the room with 400(+) sq. feet I am listening with the volume between 9 and 11 o'clock - depending on my mood. But never any louder. Important notes: 1. no monoblocks / preamp, amp is PtP integrated; and 2. speakers are "crossoverless" design. Can't be more "less..."
Speakers are just a part of the equation. Whatever you choose, it will depend on the rest of your equipment and the room. All those speakers have a high sensitivity - which is good if you are going to drive them with SET amplifier. However, look the graph of resistance across the range, that should tell you what can be the best for 300B, 2A3 or alike designs. Fairly flat 6 to 8 Ohm is great, and avoid spikes. If you have PP amp, 30 wpc and more, wild resistance will progressively diminish as a deciding factor.
Ultimately, you are the deciding factor.
To round up my thoughts:
1. graphs, despite being great starting point, can not tell you what will happen in your room, with your equipment.
2. speakers are equally important as the amplification, as the source, etc. You are listening your entire system, from the stylus to the room treatment, and everything matters. Yes, cables too.
3. as a system progresses up the quality ladder, so is the resolution - but everything depends on your taste. Same as the good camera / lens combo: it all depends on what are you trying to capture - do you want to see that beautiful moment of a butterfly landing onto the flower, or to see the trace of its fart while doing it...
