These are some speakers I found that match the classic 900 extremely well
They also fit the 9000 and 9000 cs very well

Will fit the front or the rear without any additional effort
I'll install them for you if you're local to me .. or fancy the effort of visiting for a coffee to talk classic 900's
Beware I have got a PhD in boring for England on the subject
  They will update the sound of even the oldest stereo
  Not one complaint about my speaker choices .. (perhaps I know a bit more than I thought)  20 sets sold in total

THESE WILL FIT IN A C900 WITHOUT FOULING THE SPEAKER GRILLES

4" (10 cm) 2-Way Coaxial Car Speaker
160 watt
High Grade Glass Fibre Cone
High Grade 1" Kapton Voice Coil
Multi-Directional Neodymium Tweeter
4 0HM Impedance
Sensitivity: 90 dB (1W/1M)
Rubber Magnet Band
Chrome Finished T-yoke Cover

Fits all models Convertible (front only) Hatchback Sedan 2 3 4 and 5 doors (front and rear)
 1979 - 1993
SAAB 9000 AND 9000 CS CD AERO GRIFFIN
FRONT ONLY

best wishes Steve