Furniture, wall materials, your position and the position of the sub can affect the subs response and what you hear/feel.
This is my easy setup method:
Play some bassy music and walk all round the room.
Find where the bass is at its strongest or very near its strongest if the best position isnt convenient.
Place the subwoofer there.
Play more bass and find more spots of strong bass.
You can choose any of those positions (including the current one) as places to put your sub and your seating.
This will give you a good starting point to get it setup optimally.
If you are getting too much boom at a few frequencies, try some of the other positions (that you found earlier) for your seat or sub.
Also try pulling it away from the wall(s), this will make its resonant frequency with the room on that (those) axis lower, hopefully reducing the boom enough and will give lower notes a slight boost too.
If you cant get enough deep bass, put the sub in a corner (there should be one or more good corners you found earlier), far enough away from the walls to prevent boom.
Play about
If you are serious about getting the bass right, you may have to rearrange the room around your hifi.
This is what I do.
Try moving furniture away thats near the sub if you are still getting too much boom.
Use pillows, cushions and quilts to dampen resonance temporarily. This will let you know if its worth investing in some damping material.
I have 2 cushions down the side of a speaker that is quite close to a side wall, solved my boom problem.
I forgot another really good one!
Get a solid wood bread board from Ikea.
The glued wooden blocks version is fine (what I am using).
Stand your sub on it with spikes and the bass should deepen and tighten up.
I cant recommend this enough.
If this works well for you, you can buy isolating platforms but they cost a bit more than a breadboard and I dont know if they will be better.
Oh yeah, dont forget to set gain and phase correctly at every position you try the sub in.
