Try a 1458 instead. It has the same pin-out and is drop-in compatible with a much higher output capacity. I mention this since it appears that you are feeding a lower impedance than what it is capable of driving since that is the waveform that one would expect if doing so. What are you driving? I substituted an LM380 as an output AF amp in the V4 and it does very well. No pre-amp needed.
73, Don
Don, ND6T