Am I the only one who thinks B&W is mid-fi?

You are to be commended, Musicbuff, for putting together a system that you find satisfying for $4500. Many here on Audiogon have long passed that dollar landmark and are still looking for the happiness you have found in your system. Congratulations!

Joeb, Live music is a great reference, but it pretty much works best if that live music is un-amplified. You've undoubtedly heard the cliché: 'Live, acoustic instruments in a real space,' (paraphrased). There is a reason why live, un-amplified music is so popular for evaluating audio gear--at least in theory: it reduces the variables to a manageable level.

I agree that live, arena rock shows and even live blues at smaller clubs almost always sound worse than studio efforts. But I don't believe that's what most folks are talking about when they compare a given system to live music. Speaking for myself, I mean live acoustic jazz, orchestral, and chamber music.

I think the theory is that rock/pop involves electronic amplification in the studio plus the reinforcement of giant PA systems at a live show as part of the total sound result; therefore, there are too many unknown variables involved. This makes a given audio playback system pretty difficult to compare to anything else as a reference other than different audio gear.

Unknown variables also enter into the equation with acoustic music: hall, mic types and placement techniques, recording methods, mastering, etc., but those all exist in electronic-based music as well PLUS all the vagaries of the intervening electronics. You can usually readily recognize whether system X or Y comes closer compared to live acoustic music or not. This seems to be a bit harder with "originally amplified" music compared to a live show with yet more PA electronics and speakers.

Some people only listen to electronic-based or music. Totally cool. I like rock/blues/pop, too, and I use it to evaluate gear: macro dynamics, pace, bass speed, and 'slam.' But I don't stop there; the heart-of-the-matter evaluation, for me, has to be done with acoustic music compared to live as a base-line.

Tvad, I think one eventually gets to the point in audio where happiness has more to do with what's going on inside your head than outside.

After talking with other people, here's a ththought about Nautilus. First I get don't measure a spike at 80 and 10,000 hz as Mikez says. And the BBC dip at the presence region is measured anechoic and may make actual in-room response more flat. In any event, compared to Merlin VSM, instruments and vocals aren't as actile like they are right in room. So I don't know if the frequency response is the reason.
But B&W runs the midrange into breakup because of the high crossover frequency. As shown at the B&W site, the driver produces a broad band type of pink noise at breakup which may raise the noise floor of the speaker.
I thought it had to do with the compliance of Kevlar vs metal making the fuzzy sound. But it may be the breakup mode instead. So the weave of the driver reduces harmonic distortion to 1% or less but converts the distortion into "pink noise".
I was disappointed to hear PSB Image 4T's sound clearer than my B&W. Clearer but not as clean. Meaning there was more distortion. So maybe it's a tradeoff. Thiels are clear but Stereophile, for example, always seems to find some fault. Maybe poor design or maybe the clarity is a double edged sword.
I realize many people find B&W bright. This may be a setup thing. They are bright compared to Dynaudio, Silverline, and Reynaud. For the record, I have never had anyone ever complaint my Nautilus is bright. On good recordings, there are sort of bland. But they make poor recordings listenable and still invite me to listen into the music which is important to me. Auditioning other speakers dealers have complained about my bad recordings but they actually sound okay on B&W. But maybe the "mid-fi" sound could also be because of their lack of razor sharp clarity and/or distortion.
But I find they are less bright than other studio monitors, other than ATC, Alesis, and KRK. Talking with a recording engineer, he told me he'd rather have a bright speaker and eq. it down than try to brighten up a dark speaker. So for real studio monitors brightness is a better alternative.

The only B&W speakers I have used are a little set of 550 (6.5 inch woofer in sealed box and titanium dome tweeter). These were when my living accomodations were greatly downsized. I now have an array if three MG 1.6, plus other stuff, and I like that setup.

However, I must say that those little boxes reproduce one thing, a violin, better than any speaker I have ever heard.

Possible reasons are (a) The baffle size is just about the same as a violin. (b) The Titanium tweeter has a slightly metalic sound, that resembles the violin's metal A and E strings (the two highest of four).