Hey. I know I've very late to the party... by a year... but I read something in the posts above and wanted to comment. When it was noted that your original antique wood floor had NO subfloor, and that it was simply nailed directly to the beams below, someone commented that "it was never intended to be the finished floor". I beg to differ. In my area of the country there are many antique homes with no subfloor.
I live just outside Charleston, South Carolina, in a 2 room over 2 room, balloon framed servants cottage, with double front doors, built in 1870. All of my floors are pine. One room was more for guests, funerals, sitting in on Sunday and such, and has tounge and groove flooring. The other room, which was used quite hard, has random width plank flooring. Neither room has a subfloor. All of the flooring is nailed directly to the beams below. The cracks in the flooring on the plank side, are getting pretty large now. None of the flooring squeeks, and is only "spongy" in two spots: one 2x2 foot space at the foot of the stairs, and on some very small random spots of a few boards near one window where beetles munched on the wood (and it was treated) years ago.
As a side note, there is insulation under the house (and in the attic) but absolutely none in the walls. Because the house is balloon framed, we have installed "fire breaks" in the wall cavities.
Not everyone could afford to have finished floors in their homes back in the day, and they knew it ahead of time. One clue to this will be the ORIGINAL interior door frames. If they are cut and installed all the way down to the existing floor, then that's most likely what they had planned all along. However, if the ORIGINAL door frames are elevated off the wood floor by at least 3/4", they intended to go back and add a second, "finished" floor on top of that subfloor. Back in the day they didn't go back and chop off door frames with a jamb saw like people tend to do now. They planned ahead. :)