Success! (and nobody got hurt)
Arrived at the sellers house, to see that he'd backed his trailer right up to his extra-wide basement door. There were four of us there and although we all groaned while picking it up, we managed to lift it up and onto the trailer. That turned out to be the hardest part of it all (and thankfully, was at the beginning when we all still had energy).
When we arrived at my place we backed up his trailer right to the front door. Laid out plywood sheets on the ground to help create a ramp to get us over the 1" lip at the door, and laid a moving blanked on top of that to reduce friction. Blanket would bunch up, but it was easier to slide that across than to lift the cabinet up and in over the lip.
Once inside the door, the cabinet was wrapped in another moving blanket, to help protect the walls as we go down the stairs. Should we end up rubbing against the walls on the way down I wanted it to be something softer than the 3/4" plywood rubbing against the drywall. That was then wrapped by four sets of red tie-down straps, each rated for 900lbs.
The tie-down straps were then all connected to a rope hoist by a quick-link (rated for 1800lbs). Hoist itself has a 1000lb rating on it (and was on sale at Canadian Tire last week for $13.99). Other end of the hoist was tied off to the tow rings on my brother in-law's trailer hitch with a tow rope rated for ~2400lbs. Had enough tow-rope that we looped it and tied it off four times.
We all stopped, thought about it again, and felt comfortable with what we were about to do. Rope hoist is supposed to give a 7:1 lift ratio, which should make the ~600lbs feel like ~90lbs. Still heavy for a controlled lift down the stairs, but far more realistic than the full 600lbs.
Then, the moment of truth... pushing it over the edge of the top of the stairs. Nudged it along inch by inch to the edge, and watched it slowly tip upwards and start to slide down. Hoist worked
awesome; brother in-law said it was minimal effort to slowly let the rope out while the other two of us guided this beast down the stairs. At one point, he actually pulled it back up a few inches as he wasn't sure if it was stuck or not.
Once down into the basement, we rolled it over to its final resting place, and plugged it in. And yes, we managed to do this
without taking the screens out.
Definitely a success.

Although I was a science geek in high-school and know all the physics behind pulleys and hoist systems, I was still surprised how easy it made this. Wouldn't imagine doing it without several other people on-hand to help guide and lift those few little bits here and there, but for the trek down the stairs, the hoist sure made this the easiest haul down the stairs of any cab we've moved.
w00t!