This might not help....
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Had a chance to put these head-to-head last night at band practice.
(Yes we have a killer jam spot in the singers basement.)
Set the 550 up as seen in the pic. Everything flat. No contour. Gain at Noon. Master at 8:30.
Barely touched it all night. In the end I gave the Upper Mids a little push to cut through.
Sound was full and rich, punchy with clarity yet a little gritty too.
Didn't even need a pedal all night except for some chorus for one song.
No clipping - even on my hard hits when I hit some shots with the drummer.
As per earlier feedback, it just seemed to compress those hits a little and get a little grittier/unfocused
I switched back and forth for most of the night - but in the end - the 550 won out and I just left that plugged in.
The M6 was more aggressive, focused, punchier and stayed clean longer. Also a great sounding amp IMO.
The 550 had more depth and richness to its tone which I really liked. 3d vs 1D if you know what I mean.
The 550 fit the band better. In the mix while still cutting through - versus the M6 sitting on top of the mix.
I got a killer deal on the M6 so its not going anywhere for awhile.
The competition will continue....