I have Wizard back boxes on my 2.4. They fill the gaps nicely, not Subaruish at all. They give a nice deep sound, I have a sonic boom at about 3000rpm which I love even though it only lasts a second. Absolutely no drone whilst cruising so don't be worrying about that. It's barely discernible from oem whilst cruising and a couple of times I've turned my head in my local town to see what the car is with the nice sound only to see its my wife driving the 159.
I have a couple of vids on YouTube, just search Alafa 159 2.4 wizard back boxes.
I had mine fitted at Autolusso, but anyone can fit them, just make sure they line them up properly so they sit central in the bumper cut outs. They're around £250.
GT 3.2 V6 - Q2, KONI FSD & EIBACH PROKIT, GTA REMAP, MICHELLIN PS3's, BMC CDA
159 SW 2.4JTDM TI - ALFATUNED, WIZARD BACK BOXES, KW V3 COILOVERS, NOKIAN WR A3/MICHELLIN PS3, PCA DYNAMICS ALIGNMENT.