Line Out - Line In - Line Out - Line In - Line Out

I agree with your idea of using the Phono Out.

If the Phono Out of your preamp indeed represents the output of your preamp's phono stage, presumably the Phono Out puts out a line level signal (unimpeded by the volume control) of whatever you're playing on the turntable, all the time. This would be appropriate for the equalizer. Accordingly, I would connect the Phono Out to the input of your equalizer. I would then connect the output of your equalizer to an Aux input on your preamp. To play music using your turntable through your equalizer, I would specify the Aux input on your preamp's selector control. Of course, this won't help if you want to equalize another source signal.

I also agree that there could theoretically be some degradation of your signal from the conversion to digital then back to analog. Hopefully, this won't be really noticeable. The equalizer may have a defeat switch so you can A-B compare the sound with and without the conversion process. If the change in the sound quality bothers you, I would send it back and either do without equalization or try an all-analog equalizer. (There are quite a few to choose from.) Good luck.

I called DK Design last week. They told me that it is possible to connect an equalizer to the amp. But since this phone conversation happened before I had an equalizer delivered I didn't ask for details on how to do it.
Now I need the cables to order (one end should be RCA, another Balanced) and I am curios if I should proceed (if I find the way to connect it) or to return EQ back to the store.
While we are on this topic... I read user manual of this Behringer DEQ2496 equalizer. Neat device indeed. But there is "but"...since it is all digital it must first convert an analog signal it gets from (in my case) from turntable into digital. After it DIGITALLY makes some adjustments to the sound it converts it back to analog. According to user manual it does the conversion at 96 kHz sampling rate. I wonder if the sound noticeably degrades from such analog-to-digital and digital-to-analog conversion at this 96 kHz sampling rate. It worth to mention that CD audio quality is 44.1 kHz. And with 44.1 I can definitely hear the difference between CD and analog (vinyl). Well...96 kHz is more than twice higher rate... but still...it is SAMPLING. And every SAMPLING is DEGRADATION.
I wonder how much noticeable to human ear this degradation/loss of quality is.

If your preamp lacks the capability to monitor a recording while it is being made, then you may safely forget the tape loop idea.

It looks like the amp doesn't have Tape-In and Tape-Out connectors. At least there are no connectors named like this. But it has Line-Out connector. Which delivers a signal out. And this signal is not controlled by volume knob. I am sure I can use this Line-Out connector to connect a tape recorder. So may be it is subsitute for Tape-In? It also has Phono-Out connector. Which is probably output connector of build-in Phone Stage.
I am confused. There are less than dozen connectors on a rear panel and I can't figure out how to use them properly! Strangely but the user manual does nothing describing them.
Not too bad for $3000 dollars device!

Look on the back of your preamp to see if it has tape-in and tape-out connections. But instead of putting a tape deck there, substitute your equalizer. Then select the tape monitor on the front of the preamp to listen to the signal passed through your equalizer.

What is tape loop?
I noticed that phone stage of amplifier has a line out too.
It is called PHONO OUT. May be I should use it to send a signal to equalizer? And output of equalizer back to AUX of amp?

Connect it to the tape loop of the control preamp and listen through the tape monitor?