I need some advice.  Here's the setup:
Mac Mini 2012, mostly 320 kbps MP3s ---> 3.5mm headphone jack to RCA ---> 
Onkyo TX-8555 --> 
Audioengine P4 speakersThe Onkyo receiver doesn't have digital inputs, hence my less-than-ideal analog connection.
Do I need a DAC?  Does it matter that I'm using lossy source material (i.e., would a DAC not add as much punch to lossy material as it would to lossless)?  Would a cheapo DAC help much (like 
this or 
this), or do I have to get into the $100+ range to make a difference?  This is a ~$200 receiver and ~$250 speakers, so I don't want to go crazy here and add a $150-200 DAC if I don't have to.
As an aside, I have a Music Hall USB-1 hooked up to this rig as the main source and really like the sound.