What happened to the loudness control?

That is what a loudness compensation control is, an equalizer, but variable with the volume. One hopes that it complementary to the frequency response variability of the ear according to volume.
I use the variable loudness compensation control on my Nak 630 pre-amp by setting the compensation at 0 and the main volume control for a natural sounding volume of about 80db or flat frequency response. When I want to reduce the volume to more background levels, I use the loudness compensation control. The main volume setting remains the same.
Works great! I think that the loudness controls on the old Macs worked in the same fashion.
One button loudness compensation is next to useless because it is fixed.
Bob P.

The reason the loudness control was dumped is simply that no one has ever come up with a circuit that worked 100% of the time at 100% of the volume settings. Consumers eventually figured this out and preferred not to own it.

Spatialking...No one uses 100% of the volume control range! The twin control scheme described by inpepinnovations@aol.com works very well.

Eldartford: What you wrote isn't exactly correct. Although few folks will turn their volume up to max, there are ocassions when that becomes neccessary. I have experiemented with amplifiers with sensitivities so low that my volume control was at max with some recordings.

But, the reason I stated 100% of the range is that the loudness control is a relative to the volume setting. By design, if it can't function properly over 100% of the range, the circuit is flawed by definition.

Secondly, all preamps have a dual knob loudness control if they have a bass control. Simply turn the bass up slightly to your preferece when you turn the volume down. At least you will get the proper amount of bass. Please understand the loudness control is simply nothing more than an attempt to eliminate one additional tone adjustment. It isn't much effort to tweak the bass control a smidge.

Lastly, the Fletcher Munson curves are an average, they aren't correct for all humans. Consequently, even if the loudness control followed the curves perfectly, it would still be wrong for many, if not most, of us.

Spatialking...Turning the volume all the way up is not using 100%of the range. In practice 50% of the range (eg: from 40% to 90% would be typical) and with a two knob loudness circuit tracking Fletcher-Munson very accurately over such a range is not a problem. Of course F-M is an average, but it is better than nothing. A bass control is NOT the same thing as Loudness, although people have to make do with it these days.