Kev and Joe -
I think you guys are approaching the same subject from different directions.
Kev - what you're referring to in your most recent post is the sound that they're after during the recording process
Joe - what you're talking about is how it sounds coming out of the speakers.
In this case, I would kind of agree with both of you. Jazzers, Classical-ites, Rockers and all others (if they're serious about music) all want good/accurate loudspeakers. They may not understand what this truly means, but they still want it. This is why Bose is still in business - people *think* they're accurate.
Of course, Kev, you're absolutely right - certain genres prefer to have their sounds recorded differently - more compression, less compression, more bass, linear bass and so on.
Now, back to the monitors -
Sorry to tell you - The Dynaudios still don't have enough bass energy to accurately monitor the lowest octave. I use the BM15s now quite religiously and for anything uber-critical (particularly classical stuff) I switch in my REL subwoofer to give me the tones between 5Hz and 35 Hz. This also frees up some of the wasted energy from the Dyns and the Hafler amp. (When I cross over the Dyns, I use a 12 dB/octave low cut starting at 50 Hz. These speakers sound amazing in that range and do just fine below it, but the REL is designed for these frequencies).
And I do agree with Joe - fix the mix as much as possible before going to the ME. It's not the ME's job to remove plosives or footfall - it's the RE's job.
Now, back on subject again -
Remember one fundamental rule of physics -
Sound, by definition, is air molecules moving in a specific manner to produce pitch, timbre, and amplitude.
Hence, if a cone during play has an excursion of roughly 2 inches in and out (or 4 inches of total travel) than a crap load of air is being moved. The bigger the woofer in this case, the lower the frequency possible. (of course, only if excursion distance and cone linearity or rigidity is a constant).
When you don't hear anything but you magically see your cone pop out and back then you have a fast transient acting in the lower frequency and the likely scenario is that the sound was a brief impulse at best and since the wave is too long to actually strike your ear directly (though you will obviously have air molecules in motion and thus *some* sound at the ear), it would rely on "filling up the room" or using the box that is your room to assist in reigning in that particular frequency so that the energy does not dissipate too quickly or even worse, resonate at prime or sympathetic frequencies.
If the woofer, however, is providing even front to rear excursion continually, it is in fact producing a tone. If it is a continuous excursion but you can't hear it, this doesn't mean it's infrasonic - it just simply means that the energy is minimal - so much so that it dissipates before it reaches you at a detectable level. (Remember, energy cannot be created nor destroyed - the dissipation is often in the form of heat - not enough to change room temperature, but it's there. B/C remember from physics - kinetic energy of air, or for that matter any molecules, most often transforms into heat energy. Example - try bending a paper clip over and over at the same point very quickly. Then touch the clip to the top of your lip, just below the nose (a very temperature sensitive area). You'll notice the heat generated by molecules in motion.)
If your room is technically accurate (i.e., no excessive modes or nulls) and you're speakers have the physical properties necessary to create lower frequencies, any *regular and even* cone movement will be quite audible.
As Kev states - no, speakers do not generally go all the way down as most people think. Think of the way a speaker works (slightly simplified)-
You have a magnet structure and a voice coil - a charge is passed to the voice coil which is suspended in the center of a hole in the magnet. As such, the voice coil - attached to the cone uses the principal of "opposites attract and likes repel" So, by passing a charge identical to that of the magnet, the voice coil is forced out of the magnetic field. Once it is cleared of the magnetic field, it begins it's return trip back to its resting place. (B/C again, remember rules of physics here - all actions have an equal and opposite reaction - hence, if there is a 2 inch excursion, there will be 2 inches of rearward travel beyond the cone's natural resting point - which consequently helps to make a nicely formated wave. Of course, usually another charge is passed to the voice coil interrupting its return voyage - and since energy can't be destroyed, the return trip (a physical requirement) heat is generated. The faster this happens (IOW, the higher the frequency) the greater the cooling requirement on the driver because more energy is converted more quickly.
So, in other words - once the cone has reached the point where the voice coil has cleared the magnet, it has reached it's greatest excursion point (under natural conditions - there is the possibility of excessive momentum from an incredibly loud impulse) and therefore, the cone will only vibrate/move so much air. So, these are the natural limitations of a driver's low frequency.
Now, let's imagine you have a 4" voice coil on a 5" woofer. The potential for a total of 8 inches of travel on a woofer without that much surface area would be disastrous. The cone could not remain rigid (or if it were made of a material capable of remaining rigid under this circumstance, you would find out very quickly that you have yourself a little "Voice Coil Missile") and thus the pure sound that has been passed to it is now broken up (distorted). This will destroy a speaker.
So - long story (err, very long story) short - if you're watching your woofers and seeing a lot of activity - it's not infrasonic and it ain't something you "can't hear." It is in fact sound and it is in fact in the audible register. (Unless you have a pair of monitors with 15" or 18" monitors and they're flopping around like mad.) The difference is, your monitors may not be able to produce the air movement energy necessary to make the sound "loud" OR your room is zapping the sound altogether. (Usually a pretty good combination of both.)
The simple solution is to mix with a true full range system comprised of components that are designed to reproduce their respective frequencies. Mini-monitors are perfect for stuff above say 50 to 80 Hz b/c their woofers/midranges/tweeters are designed for that (as are their cabinets - you don't want to imagine 8 inches of 12" cone travel in a 2.5 cubic foot box -- That's the definition of a ***BOOM*** box.) Subwoofers do bass and that's it. Good ones (and well-calibrated ones) do it well, others just plain suck. Be careful in your sub selection.
So, when in doubt, use a sub. If you can't - trust what you hear, not what you see. Oh, and BTW - a continuous 30 Hz tone will make your woofer move quicker than you think it will - think about it - 30 cycles per second. If you see your woofer slowly rocking in and out - you might wanna check your connections or your power line - you've got some serious issues and your speaker is potentially in danger.
I hope this helps and doesn't confuse...