The normal way to do this is, from the web UI, to select the play list and start playing from it. Then to toggle the "random" switch in the playing window.
If you have done this and there are no stray classical entries in your rock play list, it is possible that you may need to run a Scan Disk.
From the front screen and Round Knob go:
Main Menu > Settings > Scan Disk
Then, when this finishes, press and hold in the Stop/Play button util "Saving" appears on the front screen.
Then go back ad try your random.
Please understand that the Brennan Interface DOES NOT deal with the HDD music files directly. Rather it builds a Database of the HDD content (this is what the Scan Disk does) and uses the information in that to access the music files.
Thus a playlist is simply a text file with each line a pointer to the HDD.
Hope this helps,
Fred