"at least 75% of the bass track must include Rudy". "THE bass track" suggests a slightly narrow-minded view of instrumentation.
So, the question is:
If I entered - for example - a tune called "Walk On The Wild Side" (I just wrote it, catchy title, don't you think?), would I have to replace both Herbie Flowers' lines with Rudy or only the one that is a traditional bass line (in this case the upright)? Or if I entered another tune I wrote called "Big Bottom" (a bit tongue in cheek, eh?), how many bass lines have to be Rudy in that one? All of them?
I suggest that the interpretation should be "75% of one of the bass lines". Bass can be a lead instrument or a rhythm instrument or just plain old bass, and as long as "plain old bass" is Rudy playing throughout the tune, I very strongly believe that you should allow - for example - a lead bass.
If not, I will label several tracks "Extra long scale ukulele at half speed" and "de-tuned cello" and "fretless baritone guitar" in a most clever attempt to bypass this silly rule.