there might accidently be something out there that could produce tender.
Piedmontese do this with a single copy of the double muscling gene and are also lean. the mutation is way downstream in the protein compared to the mutation in other breeds, so the protein has an intermediate effect with a single copy. the other breed's mutation doesn't work (make it tender) unless it's homozygous which has all sorts of problems. the pieds have been living with this for quite a while and are not so visually hugely double muscled.
the meat is usually in italian grocery stores and is very good. the filet's are a little mushy since they are already tender to begin with. but what is cool, is that the "lesser" cuts are more tender as well.
to get tender meat with genetics, maybe 35% of the total effect with the rest being environment (feeding, penned, who knows), one must have multiple genes acting in concert to approach anywhere near the pied's.
i would try it and compare to your grass-fed steaks for reference. in general, i would say they taste less of hemoglobin (blood) than other meat. the rib-eyes are spectacular.