Hi group, I think I see things somewhat different in the black/white discussion ;) and can put some more information (not necessarily better or more correct).

Regarding what was built when by whom

The Matrix-6 and 6R, Xk, Prommer, and other Oberheim gear of that era (1986-1988) were built by Sakata Shokai and Hammond Suzuki in Japan.
The Matrix-12, Xpander, DMX, DX, and DPX-1 were handmade in the USA by Oberheim themselves. The Matrix -1000 and Perf/X boxes were also made in the States, and the only problem they've had has been buggy software in the Cyclone (buggy is a mild term).
( http://www.ibiblio.org/emusic-l/back-issues/vol017/issue4.txt)
I also read about Oberheim (or ECC or Gibson) stopping production in Japan and returning to US because of quality problems.

Oberheim sold to ecc in 85.Sold to Gibson from ECC in 88/89( )
Gibson sold also black matrix 1000 till 90/91
Tom Oberheim stated in SOS:
"For various reasons that I don't want to go into too deeply, in 1985, Oberheim Electronics became ECC Oberheim owned by my ex-lawyer. I stayed on for a couple of years, then left the company in 1987 to start Marion Systems" ( http://www.soundonsound.com/sos/1994_articles/mar94/tomoberheim.html)
He also mentioned in another place that he wasn ´t anymore involved in the development of the Matrix 1000.
In 1994 Gibson restarted production of the Matrix 1000 with a white frontplate (white, because of the white OBMX) ( http://www.soundonsound.com/sos/1994_articles/jun94/oberheimmatrix.html)


Regarding black/white models

Some people have had black and white models and there is no really difference between them. The power supply in the white models is different and I have read about slightly enhanced Software, although some people say they have black and white models with the same firmware.
I also read from someone who made a copy of his white model eprom to put it in his black one to have the same.
Apparently there are black ones with different firmware versions.

Facts, i ´ve seen some or read a lot about in newsgroups (icl. here,) or at harmony central reviews. The black model often has a power supply hum, which can be very loud because of vibrations mainly in top of casing.
Black models often suffer from bad sound chips or other defect parts affecting single voices. Maybe as a result of the vibrations
Never seen or heard about in white face or better from 1994 models, as I recently had a white faced 1988 built
Matrix 1000 in hands (at least power supply dated from 1988! and serial was 88 like: 8A0724)
I then bought a black one with badly humming power supply and 2 dead sound chips from 88, serial: 8A0521
and then finally a white one with no problems from 94: serial: 320111

I really do like the sound of the Matrix 1000!

Regards,

Ralf