Why do I get strange results (large transients, slow rise and fall times) when running a PVX-4150 at 1 V?
This is normal. Like other members of the PVX series, the PVX-4150 is designed to drive a high-impedance capacitive load at high voltage.
The output impedance is 93 ohms to match the output cable impedance. But the output capacitance of the MOSFET switches is large at low voltages, and the resulting impedance mismatch distorts the pulses. In addition, gate drive pulses charging the input capacitance of the MOSFETs results in feedthrough transients.