What is the tone difference between the Hi-Beams and Lo-Rider SS? I've read the HiBeams are a bit scooped while the LoRiders have a bit of a fatter, thicker midrange. Is this true?
This is true. The Lo Rider SS also have noticably higher tension (due, from I understand, to the hex versus round core). Hi Beams are smooth, glassy up top, feel like nickels to the fingers (very smooth), have relatively low tension, and are a bit polite in the upper mid grind area, which makes them GREAT for slappers with ash/maple instruments.
The Lo Rider SS are more in the universe of the old Roto Sounds (although not that grindy, and of course, they last infinitely longer).... grindy, tight, aggressive.
If you dig Flea, you will love the Lo Rider SS. If you dig Marcus Miller, you will IMO love the Hi Beams (using an obvious oversimplification to make my point

).
Those who dig the Sadowsky SS (Blues or Blacks) will most likely dig the Lo Rider SS more than the Hi Beams. I actually prefer the Sadowsky Blue Steels to the Lo Rider SS if I want to 'roughen up' the sound and feel of a bass a bit more versus the Hi Beams.
IMO and IME!
Edit: I LOVE the Lo Rider Nickels on an alder/RW 60's style instrument... they are 'growl masters'! They are closer in tone and feel IMO to the DR Hi Beams than they are to the very aggressive and grindy and 'tight' felling Lo Rider SS.