Cool deck!
I've toyed around with mono black, mono green and mono red Stax quite a bit. In these builds I've noticed that Smokestack is the weakest part. What I have found that I think works better is to run some Stax staples such as Trinisphere, Lodestone and Tangle Wire as disruption and then smash your opponent's face in with cheap aggressive beats. And possibly splash a second win con via two card combo such as
Painter's Servant +
Grindstone. Saga can fetch out Grindstone so you'd only need to run one main deck. Thus requiring only 5 slots to run the combo. Running
Gloom with Painter's Servant could be ugly too.
IMHO, Tangle Wire is your best Stax staple followed by Trinisphere. Lodestone is great here because he taxes your opponent and beats. Combine that with Urza's Saga and you could have an aggressive Stax deck that doesn't necessarily go for the lock.
I'm not a huge fan of Ophiomancer, Smokestack or Contamination.
Opposition Agent and
Dauthi Voidwalker for disruption + beats in these slots seems good.
A lot of people will disagree with me but I don't like Karn in these Stax decks either. He's really expensive and sets you back a turn. In Stax and Aggro Stax you always want to be ahead of your opponent. The mana curve of these decks are already super high.
If you choose to go with the Aggro Stax plan
Shadowspear doesn't suck at all. Your Karnstructs with trample will win you games against chump blockers and the lifelink will save your ass from not only your opponent but also your Ancient Tombs.