My 50W, 8ohm amp can't drive my 86db, 4ohm speakers? WTF??

OK, I am only writing this tongue in cheek.  Recently my integrated amp stopped working so while I wait for it to get repaired I just tried to use my 10 year old Sonos ZP-100, rated at 50W, 8ohms to drive my Sonus Faber Guarneri Evolutions speakers.  I knew the results  would not be stellar but I just love listening to music everyday I was hoping the Sonos would get me by for a few weeks.  Sonus Faber speakers are notorious for being power hungry and my Evolutions did not disappoint in this regard.  These speakers are rated at 25-200W, 86db, 4ohm.  At low volume the sound is alright but even at moderate levels, around 70-75db I start to hear what sounds like a slight buzzing/distortion at the upper mid range.  Its not really obvious unless you are familiar with the song.  In looking at the review Stereophile did of my speakers, the impedance does indeed drop to 4 ohm at 200hz and 4000hz so what I am hearing makes sense.  I assume this is my Sonos clipping?  If so, I am surprised that it would clip at such low volumes.  I thought clipping occurs at higher volumes?  I knew the Sonos wouldnt be able to really drive my speakers but I did not expect it to sound so bad at such a low volume.  Oh well...I will keep counting the days until I get my amp back.