Tange 900
 

Anyone know where this tubing fits in the Tange lineup? I assume it's sort of low end stuff based on some of the bikes I've seen it on, though I have also seen it on some decent mid-range bikes.

That's pretty much it, all you've just said. It IS butted chromoly though. Cheaper materials; compared to high tensile steel, mangaloy it's better. It was used on mid-range models, not the beginner bikes. Mostly used in the mid-late '80s maybe later.
I have a bike made of Tange Infinty, about the same.
Tange Prestige, Champion #1, #2 , #8 , #900, Infinity. The # ones were older.
The newest is Ultimate... here's the new types www.tange-design.com
www.velobase.com has older scans

It is nice tubing... I have a 87-88 Bauer road bike that curbs out at 23 pounds built with 900 and the ride quality is outstanding despite it's 700:23 tyres.

Cool! Are there any current road bikes that use Ultimate or Prestige or is that stuff reserved for MTBs?

I have an older (early 80s) Tange catalog on my web site:

http://os2.dhs.org/~john/catalogs/tange-catalog.pdf

"900" tubing doesn't appear in it, but there's lots of other information.

A Google search on "Tange 900" gave a bunch of hits. Apparently 900 is the second from the bottom of Tange's Cr-Mo butted tubing range so the basic metal is good and the tube set is reasonably light, if not exotically so.

It was made from rolled and welded strip, not sealmess, but that isn't a real problem as seamed tubing can be very good as the subsequent die drawing makes the seam pretty much disappear.