Still no real solution, but...
... I've learned a few things. First, the volume of the playback is determined by the "Wave" setting in the mixer on my computer's sound card. [I don't know why -- or what that means -- but it's what makes the difference.] Second, the right input on my laptop to use is the Mic input, not Line-in. Third, I had my "Velocity" and "Volume" settings in Track Properties set way too low. As soon as I set these at about 100 (out of a possible 127), the signal strength being recorded on the track was much, much better. Also, John's suggestion re: the settings on my sound card's mixer were relevant -- i.e. if the "Recording levels" setting for the Mic was too low, nothing got through. On the other hand, if the Recording level setting for the mic on the sound card is too high, Cakewalk gets a distorted signal, no matter what the "Track properties" settings.
So, I can now get a clean signal, with the following settings:
1. Sound card recording levels: Mic set at 5 out of 10
2. Track properties: Volume = 100, Velocity = 100
BUT... all of this only gets a nice sound when I go straight from the passive piezzo-electric pickup straight into the Microphone jack on my laptop. As soon as I try to put some effects upstream of the laptop (i.e. cello - pickup - preamp - echo pedal - laptop), the result is terrible: low signal strength, distortion, crumbling,... Scary.
So my question is: how can I use a preamp and effects BEFORE going into the laptop? Do I have to change the settings in Cakewalk to get the sound right, or is it something about the quality/strength of the signal going into the mic jack? Or maybe the problem is the limitations of the mic jack. Maybe I should be sending the signal into the laptop through the SCSI port,... or the USB port... Any ideas?