I would agree with Trelja the difference is probably due to a change in room acoustics. Play around with speaker positioning (if you can) and furniture placement.
IF you are looking to change equipment, I might actually start with the preamp ... when I had older, less expensive gear, changing to a better pre (or linestage in my case) made a much bigger difference than amp or source. First high-end linestage I used was a Reference Line Preeminence 1 passive - FANTASTIC ... best part is a good passive is NOT all that expensive. I think Muse amps are well suited to passive linestages if I remember correctly.
IF you are looking to change equipment, I might actually start with the preamp ... when I had older, less expensive gear, changing to a better pre (or linestage in my case) made a much bigger difference than amp or source. First high-end linestage I used was a Reference Line Preeminence 1 passive - FANTASTIC ... best part is a good passive is NOT all that expensive. I think Muse amps are well suited to passive linestages if I remember correctly.