You can get a 30 day full-functioning demo of Steinberg's Spectralayers that can separate the stems:
https://www.steinberg.net/spectralayers/
Very simple to use: load in your track, hit 'unmix song' and it'll give you stems for vocals, guitar, bass, drums, and 'other'. You can then take that drum track, hit 'unmix drums', and it'll give you separate kick, snare, cymbals and hihat tracks.
You can either rebalance these stems within Spectralayers or bounce them out to remix in your DAW of choice.
If you listen to any of these stems solo'd, you'll hear weird watery artifacts, like little bits of the cymbals in the guitar tracks, stuff like that. Shit will sound weird in solo, but if you put all the stems back together, they do indeed null with the original file down to at least -100dbfs, which is plenty good enough for this sort of thing.
If you just want to adjust some levels, turn the vocals down or whatever, no problem at all. Some eq is probably also ok. But if you're doing anything non-linear, compression, saturation, tape sim, whatever, you can COMPLETELY FUCK IT ALL UP, the phase relationship between the stems will go crazy and it'll sound really awful. So be very careful, you can't just remix with abandon like you could with an actual multitrack.
I only know any of this cause I saw a video of Andrew Scheps talking about how great Spectralayers was. I thought "I wonder if it can find any bass on And Justice For All", so I got the demo and gave it a shot. Turns out, it can. There actually is bass guitar on that record, it's just totally buried. So anyway I made myself my own special AJFA remix, with less Lars, some Jason, A LOT more James and a little echo for poor bone-dry Kirk.
