I experience similar changes when tested on my sony daps and previously owned daps as well. What I found is with m5u I use medium gain as with other daps is always high gain, so since m5u is the most powerful dap I owned it sound too loud in high gain. Medium gain is similar to my wm1zm2 which is set to high gain.
When I set low gain in sony it also sound too soft. I think low gain is wrong it just sound so underpowered and your not getting everything your dap can give you.
I have changed my dac filter to sharp roll off, if you really want to get maximum bass and soundstage out of m5u I highly recommend it. Sharp roll off is very clean detailed and fastttt. The bass just grunts into deepest it can ever get, rumbly, physical, punchy and so fast. You get a slightly more sharper treble but mannnn its so good. If you get tired you can always change back to stock filter.
I am very curious how me600 sounds in fast roll off and medium gain
I think I’ll give up and buy me600 lol it just seems to good. I also hugely love my ie600 vest ever iem I owned so incredible for the price range and what impressive nuances and sound details it can reproduce and everything is so natural and presented just like your dap do it. This iem doesn't colour your tuning it very neutral natural like its not even in the way