I designed the Firewire Solo  for the serious songwriting guitarist. It had professional features like 24-bit/96kHz resolution and balanced outputs but it specifically didn't have things that this user didn't need, like MIDI I/O, RCA I/O, etc.

I'd heard from many guitar players that they wanted something that was professional but simple to use. Many of them still used drum machines and wanted a pair of 1/4" inputs to record them to the DAW.  They also wanted S/PDIF inputs to record digitally from guitar processors  such as the Line6 POD. Other than that, a mic input with 48V phantom power for use with condenser mics and an instrument input for recording acoustic guitars with pickups and active electronics direct was all that was required.

This product resonated with that audience so it sold very well.