With some effort, I've gotten SamplerBox working beautifully without purchasing anything. I used:
-- Raspberry Pi model B
-- 8GB SD card
-- MIDI keyboard
-- USB headset
Here's a brief summary of what I did. It took some figuring out but the FAQ contained enough information.
(1) Put the image on the SD card.
(2) Inserted the SD card into a Linux PC for editing.
(3) Enlarged the main partition to fill the SD card.
(4) Created a folder /samples/ on the SD card and put my sample sets there.
(5) Edited soundbox.py by changing the audio device from 2 to 0 and setting /samples/ as the samples folder.
At this point I turned it on with a MIDI keyboard connected and it worked, but the Raspberry Pi's on-board DAC (that's audio device 0) sounded terrible. Very noisy, and the sound cut in and out in a rapid, periodic chopping fashion at around 10Hz. To fix this:
(6) Found a USB headset and plugged it in.
(7) Edited soundbox.py again, setting the audio device back from 0 to 2.
(8) Started the Raspberry Pi again.
My USB headset (audio device 2) is very cheap, but it sounds much better than the on-board DAC. I really didn't want to buy that USB DAC from Ebay -- The five-week shipping time is just too long!
Thank you very much, SamplerBox team. You have made practising keyboard in my dorm much simpler than when I was using a DAW as a sound source.