Trying to find an amp that matches my tastes (aka death metal)

"I do tend to use mid era slayer to evaluate. The production on south of heaven is rich as hell."

Ah yes, forgot about that one! Some good old Cowboys in Hell is fun too!

And I would tend to agree with the above regarding equipment. You definitely need fast speakers that can handle the "tick" of current blast beat kick drums, along with the heavy mids of detuned guitars. Current metal is just slamming. In fact, I try to avoid a "box of chocolate" mixes when I put together various playlists. Invariably the metal stuff (Def Tones, In This Moment and a few others that are on the list) pop up at a nominal volume set for other styles and just blast overall. Good ol' metal. I usually listen to my metal stuff very specifically to avoid this. Best thing to do is walk into your local Hi-Fi store with South as Heaven que'd up and have fun. Then watch the sales person wince with your music selection. Lucky for me, my local guy is a closet metalhead and we get to have fun with some of the "big stuff". To date, McIntosh for whatever reason, seems to handle metal very well through a variety of speakers.

Get a set of powered dj speakers and go nuts.  Heavy is a pretty scooped sound . 2 ways will work fine. If you did go separates youll want minumum 75 watts tube or 150 watts chan solid state. . Why i say dj stuff is some people arent able to hear their speakers are distorting at all . I have a buddy the melts everything due to this fact . Death metal has enough rawness to it that its almost indistinguishable the destruction happening . A big set of jbl powered dj speakers will live through almost anything you throw at them.   The new ones control with an app. You should be able to destroy whats left of your ears in short order. Otherwise just go to your local electronics store on boxing day and buy some pioneer and klipsh expendable  stuff and let er rip . Ok in listen to heavy stuff sometimes aswell but not the stuff that sounds like cookie monster on crack . 

I’d personally look into the Schiit Vidar (or 2 run as monoblocks) and a Freya + with your Zu’s. The Schiit Aegir puts out 20 wpc, but 80 wpc when run as monoblocks. Keeping with Schiit gear will add synergy, and Zu spkrs seem perfect for that combo. Those or Tekton spkrs would work perfectly for what it sounds you’re searching for. Tekton Pendragons have extremely tight and focused low end. I listen to all types of music, but grew up on and love punk rock and metal too. A good EQ such as the Schiit Loki would come in handy too. EQ’ing that type of music can’t hurt. No need to be a purist, IMO at least. A little EQ never hurt anyone, and if you can enhance your listening pleasure with the use of one, more power to ya! Good luck, buddy!

EQ is a great way to destroy sound quality. Tube buffer, etc. Kiss definition goodbye. You have been warned. 

For cleanest path I recommend going Integrated DAC direct to amp(s).  I use such a setup lots and it is stunning. 

I had a pair of Anthony Gallos Ref 3.1’s. When hooked up to both a class D W4S ST-500mkii and Gallo SA amp (dual amping dual voice coiled sealed 10" woofers), metal was amazing. Low volume pant leg movement from bass drum and bass guitar plucks were physical felt. Sharp imaging and drum/bass definition came from Dirac Live from the PC. Exodus sounded better in my home than at the Nokia show in L.A! So much cleaner with attack and accuracy - amazing. South of Heaven and Justice for All faster and tighter than ever before. Electric bass lines were so much easier to discern and follow popping hard at moderate volumes.

Eventually I moved on from the solid state/class D rock system and moved to a Line Magnetic 48W SET tube integrated with Tekton Double Impacts. Interesting find was how real the snare drum and cymbals became after cleaning up, re-clocking and galvanic isolating the usb signal feeding the dac. The drum kit definitely pops in metal as the only acoustic instrument. The system now plays metal no problem, 90% as good in the bass drum range and more realistic everywhere else, including all other genres of music (except rap). I laugh when I switch from Sara Vaughn to Exodus, that LM508 is insane.

Long story short, for metal, feed lots of power to full range towers with large quality woofers. Use the best calibration you can, (mine exits the pc digitally corrected) and feed a squeaky clean, clocked signal to a good dac. Watch out for cabinet resonances from the bass and get some after market footers to retain mid and high driver accuracy.

Given your set up, bass will sound better with more power, but your speakers are not the deepest reaching either. If you want to feel like your at the show, you need subs or well powered large woofers in your mains to get well defined bass down to 20Hz