I am by no means a newby to measuring the frequency response of my home audio systems, nor to using electronic equalization to alter the response of those systems. Here's a list of "equalizers I've known and used" at home, starting from the earliest, not counting tone controls on preamps and integrated amps in the 1960s - 1980s:
Cello Pallette Preamp
Tact RCS 2.2XP (first stock and then a later unit modified by Anthony Padilla of Maui Mods)
DSPeaker Anti-Mode 2.0 Dual Core (first the 2012 model, then the 2013 model)
Lyngdorf RoomPerfect (as part of my currrent Lyngdorf TDAI-2170)
As far as measuring the frequency response of my home audio systems, I've used the old Radio Shack analog SPL meter, then a version of that meter modified for flatter bass and treble response, then Liberty Instruments SynRTA or Liberty Instruments PRAXIS, and now Liberty Instruments Omnimic system.
The only fly in the ointment of the otherwise stellar Lyngdorf TDAI-2170 is the continuing problems I've had with the Lyngdorf's implementation of RoomPerfect adding extra energy in the 2 - 5 kHz region. I've tried following several different suggestions from friends and Lyngdorf, but with the same basic result: when RoomPerfect is activated, there is too much energy in that band, resulting in rather bright, aggressive sound. Yes, the bass is very nicely smoothed and the focus is much improved, but the sound from the Stirling LS3/6 + AudioKinesis Swarm speakers is just a bit too bright and edgy with this automatic correction.
I finally bit the bullet and added an old-fashioned 1/3-octave pro-audio graphic equalizer, the ART EQ355, to supplement (not replace) the Lyngdorf's automatic RoomPerfect equalization. In other words, I'm still using RoomPerfect, just manually modifying the automatic equalization results it gives by manually modifying the frequency response with the ART EQ355. As you can see from the above list, this is at least the third pro-audio balanced analog 30- or 31-band 1/3-octave stereo graphic equalizer I've used and owned.
Given the great flexibility afforded by the combination of my EVS-modified Oppo BDP-105D and Lyngdorf TDAI-2170 in this system, I have added this equalizer is such a way that I can bypass it if I want to for speaker listening and so that it never is in the signal path for headphone listening. To do this, I'm now using the Oppo as the digital source selector again. That's fine since it sounds at least as good handling that task as does the Lyngdorf. I then take the balanced analog output of the Oppo into the ART equalizer's balanced analog inputs. The ART's balanced analog outputs then feed the balanced analog inputs of the Lyngdorf. The Lyngdorf does the A/D and D/A for speaker listening through the ART.
I can also listen to the speakers via the Oppo's coaxial and HDMI digital outputs. Those outputs bypass the ART and go straight into the Lyngdorf's coaxial and HDMI digital inputs.
Headphones can be listened to via the Oppo's headphone jack. Better yet, the digital coax and HDMI signals from the Oppo feed the Lyngdorf's digital inputs, are processed by the Lyngdorf's ICC (Inter-sample Clipping Correction) just as are all signals fed to the speakers, and then exit the Lyngdorf's digital coax output feeding the coaxial digital input of my SimAudio Moon Neo 430HA headphone amp. The DAC in that headphone amp creates balanced analog output for my my Sennheiser HD800S headphones. Both of these headphone signal paths bypass both RoomPerfect and the ART equalizer.
Before installing the ART equalizer, I compared the sound from the Oppo's balanced analog outputs routed through the Lyngdorf's balanced analog inputs and A/D converter to the sound going directly from the Oppo's digital outputs to the Lyngdorf. Doing this comparison is just a matter of switching inputs on the Lyngdorf from the coaxial digital input to the balanced analog input. I compared the sound both with RoomPerfect operating and with it bypassed. Maybe there was a tiny difference, but I doubt if I'd be able to pick it out blind once I had the levels matched carefully by ear. Each input on the Lyngdorf has sensitivity individually adjustable in one-tenth dB increments, easily allowing exact subjective level matching between/among inputs.
Then I inserted the ART EQ355 equalizer into the signal path with the equalizer bypassed with its built-in direct wire bypass. Still no difference. Putting the ART into the signal path with all the sliders centered may have flattened the soundstage just a tiny bit, but still this is a very tiny difference if it is really there at all.
With the ART's sliders adjusted to eliminate the 2 - 4 kHz emphasis caused by operating RoomPerfect (I used the Omnimic system with 1/6-octave smoothing and 2 dB per division frequency response display to measure this) and with the gains carefully matched by ear again, the only difference I can detect is a matter of frequency response. That difference is all in favor of the ART in the signal path. The excess brightness is gone and the bass sounds fuller as a result.
I'm very impressed with the ART equalizer's transparency in this high-resolution system. The ART adds no electrical or mechanical hum or noise that I can detect. This is all the more impressive since the ART costs all of about $200 delivered from any number of sources--I got mine through Amazon. It looks good and has substantial heft and the sliders have very smooth motion. The range of EQ is adjustable for either plus or minus 6 dB or 12 dB; I'm using the 6 dB range since that is plenty. As I mentioned, it also has straight wire bypass buttons which allow signal to pass unaltered even with the ART's power switch off. Other features include both balanced ins/outs and unbalanced ins/outs on RCA jacks, unusual in a pro-audio device. The non-defeatable high and low pass filters can be adjusted so that they do not interfere with measured response in the 20 Hz to 20 kHz range per Omnimic. The ART also has a ground lift switch as well as a grounding terminal. Nice feature set and no distracting flashing or overly bright display lights. I would say that sonically it is at least as good as the now-long-out-of-production Audient ASP231, which was previously the most transparent graphic equalizer I'd used and which cost $1,400 new years ago.
As far as the set up of the ART EQ355 is concerned, you'll need to add four little vinyl or rubbery feet to the four corners of the bottom of the chassis; it comes with rack-mounting ears (19" wide) and no feet. I am using Blue Jeans balanced audio cable with Neutrik XLR plugs on both ends. All connections are treated with Deoxit Gold G100L brush-on contact enhancer. I do not use the ground lift feature. But I have added an EVS Ground Enhancer attached to the grounding screw on the ART. That seems to focus the center stage nicely, eliminating just about the only remaining tell-tale audible difference between the signal path through the ART and the digital signal path which bypasses the ART.
In fact, the ART equalizer sounds so transparent that I would not hesitate to use it in the signal path to my Sennheiser HD800S headphones to adjust the response of those phones. It is a simple matter to do so and I've tried it; I just choose the balanced analog input of the Lyngdorf, which would then feed the ART-equalized signal to my headphone amplifier. So far, however, except for lacking the lowest bass octave, to my ears, in this system, the Sennheiser HD800S need no such frequency response adjustment. Thus, I continue to drive the Sennheisers from the unequalized coaxial digital output of the Oppo, the Lyndorf performs its magical Intersample Clipping Correction dynamic range expansion on the signal, and then the DAC in the SimAudio Moon Neo 430HA headphone amplifier does the D/A duties before passing blissful balanced analog signal to the Sennheisers.