I just wanted to post a note to the forum to say that I have SamplerBox running on a Raspberry Pi Model B with 512Mb RAM. This is one of the older, slower models.
I'm using a USB DAC PCM2704 (Same as the one Joseph specifies in the parts list).
To test the latency I used Sonar on a Windows PC to generate MIDI Notes through an RME Digiface interface. I then sampled the audio being generated by the Pi back into Sonar.
My audio interface is running at 48KHz with a buffer size of 64 Samples.
I measured the total end-to-end latency as 2,200 samples, so (2,200 - 64)/48000 = 44.5ms
This is quite a lot worse than the 11ms Joseph is getting with the Raspberry Pi model 2, though it's good that it actually works!
Loading the GrandPiano sample set from a USB 2 memory stick takes around 20 seconds.
I built the MIDI interface on a Humble Pi v1.3 prototyping board, which fits neatly on top of the Pi. I haven't at this stage bothered with the LED Display or the buttons.