The NiceHCK M6 with the stock filters is very boomy/woolly at the midbass. I'm a basshead and I like my bass any day (for quantity), but even I thought with the stock filters, the midbass was too prominent and it eats into the mids and treble. Even though there are 3 stock filters, they are pretty gimicky and don't really change the bass frequencies much IMO.
But with the 3rd party BGVP filter for a few bucks, it lowers this midbass hump and gives the mids/treble more breathing space and the tuning is much more balanced.
Overall, the M6 has pretty thick and slow note weight, but still for a hybrid, its timbre is pretty good. Technicalities are good though there is a higher treble roll off, so it's not for trebleheads too.
If u are searching for the 3rd party BGVP filter, just type "BGVP filter" on aliexpress, many shops are selling it. Just get it from the cheapest/most reliable shop. It looks something like this:
Though with this coronavirus thingy going on, I think there might be delays when ordering with Aliexpress, maybe check with the rest who have recently ordered stuff. FYI, there's usually a big Aliexpress sale sometime in March, not sure if it will be on this year with the virus, but I got my M6 at $73 USD before coupons during the 11/11 AE sale (usually it is 87 - 100+). With coupons it was around 65 bucks. Supposedly the M6 is an OEM version of the more expensive BGVP DMG, and is cheaper by $30ish USD, I like it so much I actually sold away all my KZs and TRNs after I bought the NiceHCK M6.