I have a set of them on a Squier Telecaster bass, the one with one pickup and the three position switch: dub/V-T/baritone. They sound absolutely great: round, full, defined. Remember, Rickenbacker flavor is the synergy of everything that makes up the bass: woods, pickup position, etc. The strings simply complete the package. But just like having the right bow on the right gift box, if you have the wrong strings, it can sound bad.
Digression: Fender flats are some of my favorite strings. The current version was designed to work on a classic P-Bass: alder body, rosewood fret board, etc. And they do sound great on the one bass I have with that combination. OTOH, they sound nasal and pinched on an Ibanez bass I have that has the Asian mahogany body, even though it has a maple/rosewood neck. On that Ibby, D'A XL's sound round and clean. But on my Rickenbacker 4002 (yes, "2," not "1" or "3"), D'A XL's sound hollow and thin due to the scoop in their tone being in the same range as the upper bass scoop which is part of the signature tone of a maple body. So on the 4002 I have the last NOS GHS Progressives in custom gauges 45-60-80-105 I could find to bring out what that bass has to offer.
The Optima RB's, kind of like the La Bella FL's, are a more neutral string, tone-wise, and work in a variety of applications. They don't have the initial ping articulation that La Bella's due, and they may have a little more low mids/upper bass, which is why they sound good on a Rickenbacker.