The base-emitter and base-collector junctions of germanium audio transistors like AC130, OC74, AC127, 2N61, AC188 and AD162 were tried out. However, they were all only as good as the OA5.
As luck would have it, when the 2N61 was being tried out, an accidental short between its base and emitter leads resulted in a tremendous increase in signal output. The other transistors too gave identical results with their base and emitter leads shorted.
Shorting the base and collector leads gave similar, though not consistent, results.
With the OA5 in the crystal radio replaced by the base-emitter-shorted 2N61, the local broadcast station on 612 kHz is now loud enough to be heard faintly even at a distance from the headphones!