Before blaming amp, place some DIY room treatment at first reflection points to see if that clears up muddiness & lower-mids. If DIY room treatment makes it better, later on you can add name-brand room treatment which is generally more effective & better-looking, but tres expensive.
Also, did system sound better with AR preamp. My feeling is system may need active preamp & may need the extra oomph not available with passive volume control or no preamp.
Also, did system sound better with AR preamp. My feeling is system may need active preamp & may need the extra oomph not available with passive volume control or no preamp.