Bob Carver "Black Beauty 305" or the E.A.R. 890

Like the answer to any nontrivial technical question, it depends. Even just +3dB of extra headroom can yield a huge difference -- but it all depends on your listening habits (typical & peak SPL), speakers, seating, and room. If you shy away from loud volumes or have extremely efficient speakers then the +6dB itself likely won't matter, but you'd still hear a sonic difference between the 2 amps. Then, there are other complex interactions between amp & speaker that aren't necessarily related to the power output -- and that's where it gets tricky, and matching becomes more of (or at least partially) an art.

Nobody here can provide better than a wild guess about your power needs without the above details.

My only thought was how loud do you want to play your music? Then I remembered that some people have big rooms to fill. Even then when I lived in a relatively large home with a big formal living room, I was able to get all the volume I needed from 40 to 60 wpc, using 88DB sensitive Von Schweickert VR2s. What are you aiming for?
I will hazard a guess here and say that either approach will work, I am rooting for Bob Carver's comeback but recognize the quality of the longer standing E.A.R. amps.
I used to keep a variety of amps perhaps you can keep both.

Right now I'm powering a pair of B&W 802D's but I have a pair of Magico S5's on order. The S5's are rated 89dB where the 802's are 90dB.

How big is "big?" The room is about 24' x 27'

I don't have a problem filling the room with a single E.A.R. today (70wpc). When I think about the formula that doubling the power brings "just" 3dB more... given that dB is a log scale doesn't than mean 30% louder?

I don't listen to much music at high volume - so, the question is as much about the "quality/qualities" of the sound.

With the Magico S5 you're REALLY going to want more power than 70 Watts. They're nice speakers and I've heard them do OK with 180 Watt/ch tube amp on the 4 ohm taps, but when mistakenly moved to the 8 ohm tap (reducing power and increasing distortion, i.e. sort-of simulating a lower powered tube amp), the result was awful.

The Carver 305 should have enough power to do just fine. However, the S5's 89dB rating can be deceiving in that it's a sensitivity spec and these are 4 ohm speakers; tube amps will only yield 86dB/Watt here. A high-end SS amps will typically "double" (not really, but close) its power into 4 ohms; thus, the nature of this speaker works with SS and against tubes. I'm sure they were designed with high-power SS in mind. Also from what I heard, they'd probably benefit from higher damping factors as well -- again, lending themselves to high-power SS.

In summary, with S5 I think you'd have a bad match with the EAR and a potentially good-to-great match with the Carver, but other (SS) options may be even better. I'm a tubes guy, so it takes a LOT for me to consider SS options over tubes, but I think the S5 may be one of those special cases.

Also, your 24' x 27' room would be considered quite large! Indeed, I would suggest you require quite a healthy dose of power unless you *always* listen at very low volume levels. Really, your power needs are dictated more by the peak/crest levels you hit rather than your average levels -- these peaks can shoot up surprisingly high on high dynamic range material (classical), but they're typically very close to the average level on highly compressed rock/pop recordings.

+3dB is (almost exactly) twice the acoustic energy -- that's not subjective. How much "louder" it sounds to a person is somewhat subjective. Some folks prescribe that +10dB constitutes a doubling in perceived volume, others say +6dB; personally I find even +3dB to be quite significant, and +10dB sounds a lot more than just twice as loud.