If your structural report is fine (sills, beams and joists), well, welcome to old house living.

In our kitchen, across the 48" span in front of our range the floor drops 1.75 inches. Actually highly convenient, because the counter which is on an even level with the stove top on the right is fine for my tall husband, but the counter on the left is much better suited for shrimp like me.

You won't notice it after you've lived there a bit.

There are two possibilities. Either the building was originally built crooked, or something has changed, resulting in sagging floors. What you describe sounds like a structural defect to me.

Have you poked around in the basement to look at the floor supports and at the foundation? Over the time people do stupid things to their homes...mostly out of ignorance, some times out of carlessness. I know in our old house we found missing floor supports. Somebody must have decided the post was in their way so they simply cut it out and removed it. We've sice replaced it with a support.
From what I've been told by other old home owners a missing support in the basement CAN and will affect how things on other levels sit.

I agree that your problem can probably be traced to the basement, unless this sagging does not continue to the floor below. If you were to remove something, to look at the frame of the structure I would remove the wall coverings, not the cieling.

I've seen sagging joists repaired in an 1800's townhouse from below which involved replacing the ceilings. It's not a little job and I don't know how you can tell if it needs to be done but you might want to talk to a structual engineer familiar with old houses. If it's not a structural problem then I would live with it.

The floors in my 1880 townhouse are absolutely level but the side staircase was pulling out of the side stringer and listing slightly. The perfect repair to level it would have involved massive destructive work running supports from the cellar to the third floor. I opted for the 'easier' repair, stabilizing it in place, which involved numbering and dismantling the walnut and mahogony handrail system, opening each step and replacing the structural supports and putting everything back. You can't tell it was touched and I did it in time so there isn't much of a slant.

Whatever you do won't be cheap.

I recently learned something from a structural engineer who took a look at our house. He pointed out that in our house the walls on the second floor do not line up exactly with the load bearing walls on the first floor which do not always line up with the load bearing structures in the basement. He said this was pretty common in older homes. He also pointed out our floors have sags and dips where things do not line up, but are pretty flat where things do line up. He assured me nothing is going to collapse and that as long as it didn't bother us it could last another hundred years.

My house had a 6" dip in the second floor and it was structural. Some well-meaning PO had cut through 9 joists to install plumbing....not drilled a hole through, but cut notches 3/4 of the way through the joist. We did the repair from the floor below, sistering new laminated beams to the old beams. It was a HUGE job and not easy; but the second floor is no longer in danger of falling through. It was also not cheap and we did the labor ourselves. If anyone is going to try this, ask and I will share some of what we learned (the hard way.)

That doesn't really answer your question though. I sure as heck noticed the 6" sag. The 1" in sag in kitchen and the 1" sag in the hall I am aware of but I don't notice them on a regular basis as a "negative" feature of my house.

How do you know it's structurally sound? Like MDaugh, mentioned above, if the joists supporting the bathroom have been compromised (more than 1/2 cut through), then that could cause the sagging and that part of the floor, at least, is no longer structurally sound.

But since the first floor sags, too, well, I would figure if there is a bit of list in the first floor, then it's magnified when you get to the second floor, so perhaps the the joists in the basement are compromised.

On the other hand, I've been in a house recently where the floors go every which way, but the joists are fine. The floors sagged because it was unheated for several years. Still, the fix would've been pricey and the owners are living w/ it.

Jump up and down all up and down the areas where it lists and see how much it moves. If it's not joists, it shouldn't move too much. If the walls jiggle a bit, I'd investigate its soundness further. Then, I'd think it's structural engineer time. Our rates are low in this area (around $150 for a visit), but peace of mind is worth it and if it saves you thousands.....