I've been playing around with this in Audacity for a while, and after about 10 different iterations, I think I've finally got it as close as I can for my tastes, as far as just basic level adjusting goes.
The center channel has literally nothing at all a good bit of the time, but quite often does have just a bit of drum bleed, a tiny bit of what sounds like direct injected bass guitar, and occasionally some massively loud guitar solos or keyboard highlights. There are no vocals at all, except for maybe some traces of bleed through. I ended up going with -3dB on the center to reduce the huge discrepancy of the occasional guitar/keyboard part. I could've possibly gone a bit lower, but I didn't want to take away from that little bit of clean direct bass guitar sound, and those solo parts should stand out a little more overall, just not to such a ridiculously absurd degree.
The bass/LFE is incredibly weak to me, so I tried increasing it a stupid amount, which ended up sounding way too thud-like and boomy, just on certain sections, so I kept reducing it to where it still has some oomph without being too boomy, and ended up settling on +9dB.
The surrounds were quite weak to me overall, so I increased them until it sounded more balanced, and while the few bits that are actually back there in the surrounds sounded much better to me, the echoey/reverb stuff stood out too much, so I kept backing it off until it sounded like the best balance between enough of the actual instruments and not too much reverb at +2.5dB.
After all of these 'corrections' it sounds much better to me, but the vocals (which come mainly out of the mains but also from the rears with a good bit of reverb on them) still seem a little distant and a touch harsh, and the bottom end of the kick drum and bass guitar could definitely be more defined too, with maybe a bit of judicious eq.
I'd say that for me, what I did takes it from my original vote of 4 (probably too harsh) to probably somewhere between 7~8.
