I think I solved my own question, though I haven't been able to test it yet.
I think I found my answer in section 7.12.1 of the manual:
There's an important thing to keep in mind: the Attenuator knobs will still control what they
are pre-wired to control unless you interrupt the pre-wired connection. This is done by
plugging in one side of a patch cable to the appropriate input jack. You could also use a
"dummy" TS plug if you prefer to avoid cable clutter.
In practical terms using the same example, if you want to control the output of LFO 2 into
the Pitch 2 input:
• Att 1 > Cutoff will control both the output level of LFO 2 and the Filter cutoff at
the same time unless you plug something into the Cutoff input jack in the FILTER
section of the patch bay.
• Att 2 > Amp will control the output level of LFO 2 and raise the minimum audio
output level of the Amplifier unless you plug something into the Amp input jack
in the AMP section of the patch bay.
So I think when I'm trying to use attenuator 1 without plugging a dummy connection into the cutoff input, when I scale back the attenuator I am also lowering the cutoff frequecy so I am only hearing low frequency components of the sound. It doesn't actually sound like a filter sweep to my ears, but maybe that's because it's also interacting with the LFO in a manner I was not expecting to hear.
Likewise, I had tried briefly with attenuator 2, and it was not responding as I expected, but perhaps in a different way due to the increase in minimum output threshold.
I'll have to play with this some more but I'm certain I'll have better luck with plugging a cord into the filter cutoff or amp input.