More experimenting yielded the following results:
- All oscillator silders at 0
- All A sliders at 0, and all D, S and R sliders at 100
- Mod wheel is at 0, glide is off, LFO routing knobs are all at 12 o'clock (zero), arpeggiator is off
- Brute Factor is off
- For the filter section, Env Amount is at 0, KBD Tracking is at 0 (off), Resonance is at max, Mode is on LP, Envelope speed is fast
This, as expected, sounds like a sine wave whose pitch is constant across the keyboard but varies as the cutoff knob is twisted.
With the cutoff above 12 o'clock, everything sounds as normal. Ringing at slightly above C6, about 1063Hz. Move the cutoff knob one mark to the left (anti-clockwise), and we're about just above C#5, around 572Hz, with a 1dB drop in amplitude. HERE'S WHERE THINGS GO WEIRD. Moving the cutoff knob just one more mark to the left (so two total marks anti-clockwise from 12o'clock) and we lose all amplitude from the sound entirely. This seems to happen right around C#4, or between 375Hz and 380Hz.
What's going on here? I am totally stumped. I know that I used to be able to get some great deep kicks from my Minibrute, but that is impossible if the filter won't resonate below C4. As I said earlier, cranking the Brute Factor gives me some bass frequencies back, but that wasn't necessary in the past—in fact the Brute Factor was a great sound-shaping/tweaking tool once I already had a decent kick dialed in.
FWIW, haven't been mistreating or gigging with this keyboard. It's been sitting on a desk its entire life. If something happened to it physically without my knowledge, it didn't affect any other sounds. All I know is that this is the first time in a couple months that I've tried to dial in a Kick patch and the first time I've ever been completely unable to get the frequencies I need.