i once had a real old and beaten up 166 , i sold it after a few weeks, extreme hiss and distortion was what i had instead of nice compression, but i guess that unit was half-dead,
after that one i got me a 166a which is an OK compressor, but i got rid of it since i find that the 166xl sounds more clean, and has 'manual' attack and release buttons,
but now i'm thinking of selling the 166xl and replace it by a 160 (mono),
after i started working with the (cheap) 163x compressors i was amazed with the fat pumping effect you can achieve with them,
and since the 160 is supposed to be higher quality i want to see how this machine sounds like on kick and snare
BUT, those 163x compressors have a very distinct sound and i'm into fat and agressive sounds, if you prefer a little more clean compression then go for the XL, it won't dissapoint you, but if you got an extra $50, also get
a 163x 
i think its something you should have used once in your life
