If you want to do this, I recommend that you get a large piece of oak, not small, large, and do the whole process from start to finish. Use a piece of flooring, you can purchase it at the home store, or used would be better, about 6 to 8 inches wide, and 3 to 4 ft long. I assume you have done this before, but I recommend Bona Kemi waterborne products. These are products made specifically for floors and the application is easy. You use an applicator that looks like a thin 18' roller that has a broom handle attached to it, and you apply the finish by pushing at an angle, like a snow plow. The applicator has sleeves but does not roll like a roller. They also make the stain. But like I said, be sure to do a large pratice piece so you know what you're getting before hand. You can find them on line. You might check out onlinefloorstore.com

If you are going to use a traditional oil-based stain, I recommend MinWax "English Chestnut." It is a perfect deep rich brown without the redness of mahogany, the orange tone of cherry, or the black tones in walnut. It is warm, and gives the look of old patina even on new floors.
I'm sorry that I don't have photos of the floor in our previous house that we used it on.
And I agree about making a sample, using the same material as the wood of your floors. On the other hand, if your floors are not dark now, and you are considering removing the varnish but not sanding to bare wood, try the process out on a closet floor first.

Post your question on flooring.

We had our "orange" oak floors from the 80's handscraped and refinished in Coffee Brown by Dura Seal. We were looking for a dark color with no red or orange in it.

Thanks for the tips - we definitely don't want any red undertones and I'm going to get some Oak pieces tomorrow. Thanks again!!

Here's a before and after of our floors (handscraped and Coffee Brown stain from Dura Seal):

I second the Minwax English Chestnut suggestion and also the recommendation to try out the stain on a sample board first. I was looking for a warm brown color and went through many sample boards before I decided on two coats of the English Chestnut.

Meaning no disrespect to other posters, I find (for me) that if I use a product that is made for professionals, specifically for one purpose, I get better results. I may sound like I work for Bona Kemi but I don't. I just have used it on the floors I have done and I get good results. If you are going to stain and topcoat a floor without sanding down to bare wood, they make a product for that. To kill red you have to use a stain with some green in it. You will have to order it on line or buy it from a flooring business that sells to floor technicians. If I hired a business to redo my floor and they showed up with MinWax, I would send them away and hire someone else. If you call one of the floor businesses that are on line they will tell you what you need and how to do it. Also, there are some good books you can order that tell you what to do. What ever you decide, let us know and send pictures of how it turned out.

We did all ours,we used water based poly after 20 years still look good,I wouldnt go too dark I did once I
hated it,It shows dust alot.Mine are like a honey color.