Subwoofer Frequency Response question

It could simply be room modal - this will not go away no matter where you
place the sub. Your only hope would be to add plenty of bass traps and EQ
down the peaks to make the trough relatively less big.

It also may be caused by your speakers - as the bass is omnidirectional it will
reflect off the wall behind the speakers. You get quarter wave cancellation
across the entire room.

Are your speakers on when you make this test? Or is it just the sub playing?

Just in case - perhaps I should explain what you should do with speakers...

You should try to avoid middle ground - either place your speakers within 1 M of a wall or more than 2.2 M to help reduce this problem. there are formulas to work out at what frequency you will get cancellation - remember this applies to the ENTIRE room. I put my large full range speakers into a soffit mount - only this way you can completely eliminate the problem - that is what studios do. I leave my surround satellites within two feet from the wall (they roll off at 60 Hz).

I quote from Genelec

For large loudspeakers, with very
low cut-off frequency (Active 3-way and Large systems), placed away from
the wall the necessary distance is far too long for any practical rooms. The
conclusion is that large loudspeakers should not be placed free-standing but
flush-mounted to perform in the best possible way.

The cancellation frequency is Fc = C / 4 * D.

D= Distance from front baffle of speaker to rear wall behind speaker.
C= 344 Meters/sec
Fc is in Hz.

In your case a suckout at 45 Hz equates to a Distance of 1.9 meters. And 70
Hz equates to a distance of 1.2 Meters. If your Europa speakers are in
between these two limits then it could well explain your problem. Typically
this yields a 20 db suckout. Most people have this problem as they use large
full range free standing speakers. Since most people ignore it - even a 20 db
suckout - as bad as it seems - is it not likely to ruin your enjoyment of music.
Sometimes other room modes can compensate at the listening position.
Hopefully you can see that speaker position can be important too - not just
the sub.

does the size of the suckout change predictably as Fc changes.

The suckout will be less as you get further from the rear wall (fall off with distance - double the distance = 6 db SPL drop in the strength of the rear wall reflected wave).

Bear in mind that there is more than one suckout - it actually looks like a "comb filter"...each succesive dip as you go up in frequency is lower (this is because less and less energy goes backwards as you go up in frequency - and above 600 Hz most of the energy is going only forward - this is because the wavelengths at higher frequencies are becoming shorter than the speaker baffle)

I see why you like the soffeted approach

It is not me. I simply copy what they do in the highest end custom installations in pro studios. What I have done is far inferior to a proper setup designed by an acoustician, like reknowned Roger D'Arcy but it is still a lot better sounding than nothing at all. I share this info becuase I know from first hand how important it is - I have directly experienced the sonic improvement and it a lot more tangible than your common run-of-the-mill tweaks. (tiny cups, spikes, pads etc.)

In a Mains/Subs set up, you will likely have 2 different Baffle to front wall distances. Do you know how to determine which is relevant (or in what ratio relevance can be determined) to the calculation?

You need software to do this. It is much more complicated. However, the effect with a sub is much less critical because it is rolled off above 40 or 50 Hz anyway and room modes can really dominate at such low frequencies anyway. The best thing to do with a large sub is to get a downfiring one and place it up against a side wall. This means you eliminate most of the quarter wave problems by coupling to both floor and side wall. Since it is to one side then the distance from the sub to the rear wall behind your speaker will vary - say 2.5 meters to one corner but 8 meters to the other corner - this in itself will help reduce the coherence of any cancellation or "suckout" (it is unlike the case of two full range speakers lined up symmetrically to a rear wall). Two subs symmetrically placed close to freestanding speakers should be AVOIDED - this is contrary to the popular audio myth of stereo subs (audio is full of myths). Ideally you can also soffit mount the sub and place the woofer close to the floor but it is simply not as critical as the mains because it is normally not placed symmetrically, room modes dominate anyway, and it only handles the bottom octave (no nasty lower mid bass effects).

Conversely, is the idea behind <1m to keep the suckout in the same region as bass peaks in a typical room? Given the variation in peaks from room to room, this strikes me as quite a risky approach.

I see I missed this. The idea of going close to the back wall (less than 1 M) is to push the "suckouts" up higher above 100 Hz where they become less noticeable. As the wavelengths get shorter then the suckout will be progressively less dramatic...