Let me just clear up a few things:

I'd love nothing more than 451 rims and tires. But the last time I needed 451 tires, they were nowhere to be found in Jeddah, and buying a pair of Primo Comets online cost me more than the bike they were going onto was worth. So there's that.

Not that quality 406 tires are any easier to find locally, but still, in an emergency, I can just walk to the nearest cheap bike shop and get a temporary replacement. Not so if I were running 451s. This is strictly a 406 country.

By "thoughtless tossability" I meant leaning into any corner at any time, without feeling the need to plan ahead, not throwing my bike across the sidewalk!

But did the Speed feel like a Miata and the Mu like a an old Benz? Did the Speed feel like a Benz that handled like a Miata? A 190 with the Sportline option, maybe? I'd love to have more feedback, please!

It actually might be; everything else between the Mu I'm eying and the Speed D8 I already have is the same or very similar: 52T/11-32 (8-speed) gearing - check. SRAM X7 1:1 with matching shifter - check. Double-wall rims - check. Schwalbe Marathon Racer 20x1.50 vs. Schwalbe Marathon Supreme 20x1.60 on the Speed - let's call it a check. Hence my original question: for those who have experience with both bikes, is the Mu closer to the Speed end of the spectrum, or does it lean more towards that of, say, the Vybe? (I've never tried other Dahons, such as as the Boardwalk, Vitesse, Route... etc.)

I've already thought of this, but I don't see it happening in this economy. I've already started a thread about running a pair of 20x2.10 Primos on the Speed's narrow rims. I'm thinking of buying wider wheels just to be able to use those on the Speed (I already ran them on the Vybe for a while and they're awesome.)

See above. Plus, Jeddah is more or less flat. Aside from the occasional parking lot exit ramp, I hardly ever use 1st and 2nd.

It's actually interesting that you should say this - I've been thinking the same thing. I've got four bikes (six, if you count my wife's trike and my 30-year-old Raleigh Flyer that I've got stored back home in Amman.) Since I bought the Dahons last year, I've all but abandoned my 7.3FX and 7.6FX. I'm actually thinking of selling off both bikes, instead of having them sit around, taking up space and gathering dust. For the money I expect to get for them, I could probably afford that Mu and have some left over.

You just made the whole point of my original post - I absolutely love my Speed; it's perfect in every way: it fits me like a glove, with just-right handlebar reach; it corners with aplomb, rolls like it's on butter, stops on a dime, and it's reasonably comfortable to boot - so much so that the suspension seatpost that I've been testing did little to improve the ride quality. So, what I want to know is whether or not I'd be missing out on something even better (in terms of handling/fun-to-ride factor) if I don't buy that Mu.

No need; it's just about perfect for me. My Vybe is a bit of a stretch in comparison.

Interesting, but just not enough. I guess I'll just hop over to the store again and see if I can get them to let me test the bike around the mall's underground parking lot.