haven't put any compression on them at all yet.  Like I said, all I've done so far is pan them each like a string quartet seating arrangement in the stereo image, and added some Hall reverb from the BREVERB 2 module.  And it sounds pretty good to me so far.
 
 

That may be all you need, TBH. Unless frequency collisions are occurring (which may require mirror EQ as the mix grows), panning alone may be all that is necessary.

I'd caution against overuse of compression on a string quartet. There should be a marked difference between piano and forte passages. Some guys I know think compression on an orchestral piece is evil. I'm not of that mind, but on the other hand orchestral music isn't EDM.

I try to make the forte passages hit around my RMS max target. I'll level out softer passages a little - maybe. If you listen to classical music in the car, you'll probably find the softer passages are a bit hard to hear. And IMHO that is as it should be. But that's me 😊

HI:)
 
Don't use Compression on clasical music, it need's the dynamic. For You mix at -23 dB (EBU 128), you only have to look at some Peaks, perhaps use a Master Limiter at about - 1,1 dB, (Boost 11 is OK with 0 dB Boost!) True Peak is peaking at -1 dB.
For EQ look at 110,220 Hertz, that's the Problem with Deep Strings ( Don't forget LowCuts at 40-50 Hertz), and about 5-6 kHz give a little bit for Clarity. If you like it, cut a little between 500-700 Hertz. For all, not too much, +- 3dB!
Depends on the Mics you used....
I've sometimes recorded String Orchestras, and there, as said I just have to do the Paning and all is ok:)
 
Greetz,
Bassman.