I had some time yesterday to compare a SR-Lambda NB (1979) to my Lambda Nova Signature (LNS) - both were connected to a T1 amplifier with CCS modifications and using the Schiit Yggdrasil DAC.
The SR-Lambda NB delivered superior clarity, detail, dynamics, and energy. However, the LNS had better tonal balance, extended low frequencies, and lacked the occasionally strident highs of the SR-Lambda NB.
For certain music, like classical works with abundant strings, the SR-Lambda NB may be the best Stax headphone ever (surpassing even the sr-omega, sr-007, SR-009, SR-X9000, etc.). But on a lot of modern recordings its high frequencies can be unrelentingly sharp.
In summary, the LNS is likely the most evenly balanced Lambda headphone, and the best value Stax model for audiophiles on a budget. Yet the SR-Lambda NB is uniquely capable of outstanding performance with the right musical genre and can be a great complementary headphone in a large collection.