Purchased the pair from a local car audio store - Eclipse does not retail on the internet, but more details can be found at http://www.eclipse-web.com.

I bought these speakers to replace the factory dash speakers in my 1990 Coupe Quattro (same dash cutout for all '88 - '92 Audi 80 and 90 series cars and others I believe...).  Fitment was perfect.  The tabs on the Eclipse speaker frames fit in the OE placements with no modification. Speaker magnets will clear the dash edge and ducting without interference.  No cutting, shimming etc. needed.  Installed in less than 15 minutes - even the OE speaker wire clips are the right size and shape.

Sound quality is excellent.  I had a pair of JBL 3.5" speakers that, even when amped with my Alpine amp, still sounded tinny and quiet (huge magnets, underdriven).  The Eclipse speakers sound very crisp and responsive with just deck power (Blaupunk Toronto CD unit) and I'll be
hooking them up to the Alpine amp when time permits.  The Eclipse 8233s are rated for fairly low wattage applications (20-40 watts), but I can say from experience that unless you have a high wattage amp to drive higher watt speakers (my old JBLs...) your sound quality will suffer.
Go with speakers rated for the power you can provide.

Eclipse 8233 speakers are a great choice for deck power or small amps, and a huge improvement over the stock single cone 4" (more like 3.85" odd ball) speakers common to Audis.  I've been told a 4" speaker will fit, and others have said not without cutting and modification.  Eclipse 3.5s fit great so...  Going rate is about $80 a pair.  Picked mine up on
sale for quite a bit less.  Shop around.