it is all confusing, 1, 2 or 3 numbers entered, it just generates a load of (to me) random frequencies that mean nothing and it does not explain any of it
Think I need to come back to learning about subs, when I am ready to buy one !!
Put your room length into the first box.
5m gives room modes at 34.4Hz and second harmonic at 68.8 Hz. Ignore the rest because they're too high in frequency to matter.
If you are suffering a peak or trough at those frequencies (and you are) then movement of either the sub or youself up or down the length of the room will affect a change to those modes.
Remember, these effects are localised to specific areas of the room. The higher the harmonic the more often the mode happens, but at a reduced potency. Moving a small distance may move you out of the effects of that mode, or help ameliorate the effects of a bigger one.
The same goes for width, except of course we're now talking about movements across the room.
There's not much you can do about height and trying to figure out tangential modes will make your head explode. What you are looking for is normally across the top of each table and that is frequencies that are repeated or very close to each other especially if they're below 100Hz.
Repeating frequencies either are either additive (give a really big peak), cancelling (make no difference), or destructive (give a really big dip).
For instance you have a combined peak at 68.8 & 71.6Hz which are shown in the Axial Modes of the calculator and in your REW graph. Either peak by itself is probably not quite as big as shown, but the overlap gives a boost to each. When you get your BFD it would be tempting to create a filter at 70Hz to squash 'em, but two less savage filters at 69 & 72 Hz may do a better job and sound better.
If a trough fails to disappear whatever you try, there's nothing you can do about it and certainly don't try using the BFD to boost them if they're below 40Hz. They can be caused by openings into othe rooms, chimneys or alcoves. Unless you fancy rebuilding, they're normally there to stay.
Its all about looking at different pictures to gain an idea of whats going on. I'd use the room mode calculator plus REW to do what you can in terms of positioning before the BFD is introduced. EQ is very handy, but less is always more with audio.
Russell