Equalization

@stereo5 
You are a liberated audiophile!

Indeed. It is one of these unfortunate audiophile myths that tone controls (or balance controls) degrade the sound and hence should be omitted. Makers of high margin audiophile electronics were only too happy to oblige as it saved them money.
The truth is that speakers are far from perfect, and their in-room response is even worse. Traditionally we had two types of tone controls to do something about this: the classic design by audio legend Peter Baxandall and the Tilt control by his friend and other audio legend Peter Walker. And even the Chromecast Audio streamer has recently been given a digital implementation of the Baxandal controls.
The limitation of these tone controls is that they cannot target quite narrow peaks and dips such as you encounter with in room response at lower frequencies. For that you need so-called parametric equalization. These days, with digital signal processing, that is easily achieved. One way is if you are using a computer as a source. Measure in-room response with the free REW software, generate a correction curve and apply that to the free Equalizer Apo software that you can install on the PC.  The same software can also be used to generate so called shelve filters to lift or depress response over larger parts of the spectrum. If you do not use a computer as your only source, you will need a miniDSP unit in your chain to apply that REW correction curve.
Alternatively, there are automatic systems, from the basic and very cheap Antimode 8033 for subwoofer equalization (room mode problems are biggest at the lowest frequencies), to the very advanced and expensive new Antimode X4 that will equalize not just the subs but also the main speakers, and act as a DAC and pre amplifier as well.

Most balance and frequency problems are actually produced by room anomalies. Comb filter effects, echos, first and second reflections, too much absorption, etc. A balanced program of tube traps, diffusers, resonators, Mpingo discs and other room treatments will get rid of the problem rather than covering it up. 🙈 First you need to map out the room including the 3D space to locate all sound pressure peaks that are at least 6 dB higher than the average SPL in the room when playing a test tone at moderate level. Of course there is the additional worry of the room walls flexing and vibrating like drum heads. More about that later.

This is great info.
Being a Musician (50yers+) and have done some live sound.
I have also recorded in many studios over the years is the reason I asked the question.
Studios go to extremes to "Tune' their monitors to the room and the room itself.
I took an RIAA recording course at Intermedia Sound in Boston.
Famous for Recording Aerosmith's first Album and other well known artists like Donna summer and the cars.
The Room was actually built on Truck Shock absorbers.
Although I have plenty of Music and Audio experience the Hi End Hi Fi Purist world is intriguing.I am learning a whole new concept of listening to music.

Unfortunately there is also quite a bit of snake oil being peddled. As for room acoustics and equalization, including some useful links: https://forum.audiogon.com/discussions/room-acoustics-10