There is a substantial improvement in the circuit design of the 4B-ST (the "ST" stands for Stuart Taylor, the electrical engineer who designed the new circuitry). The 4B-ST has roughly an order of magnitude less THD and IMD than the NRB, which is audibly noticeable on well-recorded CD's. To my ear, the "ST" version is also more dynamic, with greater transparency, better bass response, and improved treble.
