I am playing a nylon string guitar along with a harmonic octave generator and effects unit. I switch from clean to effect-based melodies. The Marshall amp I am currently using is unable to handle the lower range of the octave generator.
What should I look for when selecting a speaker system that can give me all the thump and the highs with clarity?
I would recommend using a keyboard amp. A keyboard amp will have the range and wattage to properly handle high and low frequencies. You could also try a bass amp, as most of them can cleanly accommodate clean high-end frequencies. You definitely won't be able to get the same sort of distortion/overdrive that you can from a Marshall but effects pedals should take care of things. If not, then you may want to run two amps and split the signal, sending the low frequencies to the keyboard or bass amp and higher frequencies (or all frequencies with bass range reduced) to the guitar amp.
An electric guitar amp is heavily biased towards midrange, normally without much bass or high frequency response. So I would 100% agree with Basstickler's response, but I think involving your Marshall will lead to muffled sound and frustration. Some bass amps (obviously better for low frequencies) can handle higher frequencies too - some even have a tweeter, but there's an element of square=peg-round-hole there. You're not really playing a bass.
You can also get dedicated acoustic guitar amps (full range) but Basstickler's suggestion of a keyboard amp is probably the best all-round solution. Loads of range and can delover a good thump.
