The Arduinoe TRE has the same System on Chip as BeagleBone Black (am3358), but it also includes an audio chip with ADC/DAC and thus audio input/output which makes it an ideal board for processing audio in real time.
Add to that a couple of mono blocks (http://www.redcircuits.com/Page150.htm) and a Manga Screen (https://www.kickstarter.com/projects/1924187374/manga-screen-multi-touch-43-lcd) , and put the whole shebang in a super fancy cabinet (http://en.wikipedia.org/wiki/Power_Mac_G5), and you have maybe the worlds most awesomest guitar amp!
The Spectrum analyzer that I have found to work the best is really just a terminal block thingy, almost like an ASCII art speectrum analyzer: https://github.com/karlstav/cava/.
Speaking of real time, the ideal way to handle real time audio on Linux is by using the Jack framework http://jackaudio.org, but I have been unable to get that working correctly with sending an audio stream to the output AND to a spectrum analyzer. You can do one or the other, but not both. The problem appears to be buffer underflow and the most likely solution is to use a real time kernel. Since the Arduino TRE is not generally available (I have a beta testing edition) the config/source/patches for a new kernel have not been made available as far as I can tell.
Jeezes, someone should definitely make this!