I found a great tool for making band limited waveforms:
https://www.futur3soundz.com/wavetable-synthesis Near the bottom of that page you will find a link to WCreate and instructions for using it. It creates .wav files with a specified number of samples (use 1024 for Preen) and sinewave partials in the magnitude, frequency and phase you want. I used the method mentioned at the top of this thread with Audacity to convert the .wav files .txt files for Preen.
I'm finding the additive synthesis possibilities with Preen algorithm 27 quite interesting. For example you can use WCreate to make a waveform with some harmonics and another one with some of the same harmonics 180 degrees out of phase. By adding the two in the right proportions you can remove harmonics from the first waveform. This can be controlled by the operator envelopes to change over time. This allows more precise control of harmonic content than the FM algorithms.
WCreate also makes awesome sounding organ tones!