Someone explain human hearing of subbass......

I remember my old Pioneer receiver having a "loudness" switch, which basically emphasized the lowest frequencies when playing at low volumes. Is our hearing not as sensitive to subbass frequencies at low volume? If so, how do you equalize this? High end preamps have no equalizer, does this mean you necessarily will not get decent bass at low volume levels?
"Is our hearing not as sensitive to subbass frequencies at low volume?"

In a way, you're getting into what is called the Fletcher/Munson Curves. The "loudness" switches can't do a very good job at it do to the variations in LF performance in loudspeakers and amplifiers. William M. Leach addresses those "loudness" switches specifically in one paragraph in "Introduction to Electroacoustics and Audio Amplifier Design" 2nd ed. p. 8 citing those reasons. The basic idea isn't flawed, but in the end they turn into generic "bass boost" circuits.

"If so, how do you equalize this?"
Can't really. A fancy DSP system might stand a prayer.

"does this mean you necessarily will not get decent bass at low volume levels?"

Not necessarily, there's alot of other things that affect good bass or make it bad. Alot of people would probably be amazed how little bass their amps actually have even though they're still rated almost down to DC.

And room's acoustic/nodes will affect LF performance many more significant ways that also makes those buttons more-or-less insignifianct.
We've done quite a lot of research in psycho acoustic response of the human ear. We did this in particular for our PARC device and BARE software (and thus focused on the bass frequencies), so I was hoping we could shed some light on the subject. However, Ezmeralda has already summed it up very nicely. Good post.
Ezmeralda may have summed it up nicely, but can someone explain to me how a properly-designed amplifier rated flat to almost DC lacks bass???? Or is that not what you meant?

Also, to answer Mythtrip's question, in high-end systems set up for natural bass response at realistic playback levels, you are quite correct in assuming that at low playback volumes the bass will tend toward the anemic. That is what loudness circuits are attempting to correct. However, under real-world conditions due to acoustic doubling and cancellations in the low bass, these loudness circuits may improve the perceived bass balance but are certainly far from a perfect solution (and there usually no cheap and/or practical "perfect solutions").

Changing the speaker's location in the listening room by moving the speakers closer to room boundaries can do a lot to reinforce low bass and is normally a good approach... Though sometimes a compromise must be struck between the best imaging and the best bass balance. Personally, I like the flexibility that adding a good dedicated subwoofer system allows. Then you can leave your main speakers where they image best and dial in the bass to taste.
Real Sub-bass if more felt than heard. Think earthquake.
I had the same problem with low frequency tonal equalization because I play my system most of the time late at night and less frequently in the day. Instead of knocking off walls and rearranging the furniture, I found a really workable solution for my system. At some stage of this crazy hobby chase, most of us will come to realize that the powercord at the cdp influence the overall sound most.

Two powercords are painstakingly chosen dedicated for my cdp. For daytime loud playing level I use the Harmonic Tech AC11-3 and for the nightime i use a slightly thicker DIY powercord "voiced" to provide that "loudness" boost in bass and suffer a tiny bit of suck-out in the mids. The rest of the system remain untouched. I found a peaceful balance in the two pc's for playing at two preferred listening levels. Playing modern Jazz & vocals at low and medium level most of the time, my DIY pc has taken a more permanent residence!

Costwise this works out very well for me. Enjoy!