PS:
"powerful" might be a less good word. Perhaps "thick",
"voluminous",
"full-bodied" fits better. Also "pressure", "punch", "a
fast
response"are missing.
IMO the "muddy" sound you described isn't (much) related to a filter
effect for frequencies for what frequency range ever. The complete
chain of a guitar amp makes the sound, especially the pre-amp and the
12" speaker/s. That's why Len mentioned to use a speaker and I
mentioned to use a real guitar pre-amp, since using a speaker isn't
always possible, e.g. in a rental flat.