No matter how good the stereo is, the acoustic interaction between the speakers and the room can sound better with an equaliser. Most preamplifiers or integrated amps have some sort of switchable loop in the signal path. The loop has to have a constant signal gain, not variable with the volume control. This is usually the tape monitor loop. When you connect an equaliser into this loop, the equaliser has a replacement tape monitor loop where you can reconnect the tape deck.
On the back of the preamp or integrated amp, locate a cluster of RCA jacks marked "TAPE IN/OUT" or "TAPE MONITOR." Plug an RCA stereo cable red plug into the "TAPE OUT" red (right channel) jack. Plug the white or black plug into the "TAPE OUT" white (left channel) jack. If a tape deck is plugged into this cluster, unplug all its connections from the preamp before you start this step.
On the back of the equaliser, locate the "INPUT" or "IN" white and red jacks. Plug the other end of the Step 1 cable into the input's matching colour jacks.
Plug another RCA stereo cable into the "TAPE IN" left and right jacks on the preamplifier, again matching red to red, white to white or black. Connect the plugs on the other end of the cable to the "OUTPUT" or "OUT" left and right jacks on the equaliser.
Locate the tape monitor input/output jacks on the back of the equaliser. Plug the tape deck's output RCA pair to the equaliser's "TAPE IN" jacks, again matching colours. Plug the tape deck's input RCA pair to the equaliser's "TAPE OUT" jacks.
Depress the tape monitor button on the front of the preamp. Turn on all components and play music. Moving the sliding frequency controls on the equaliser should now change the sound coming from your speakers. If you want to play a tape, depress the tape monitor button on the front of the equaliser to connect the tape deck.
If you have two tape loops, but you will no longer use the second one: you'll be using the tape monitor(s) on the equaliser, instead. When considering a receiver, integrated amp or preamp with an eye toward using an equaliser with it, buy it only if it has a tape or switchable processor loop. Make sure the equaliser is not set to bypass mode. Some receivers have "accessory" loops. These work the same as tape loops.
Do not connect an EQ between a preamp and amp: that will overdrive the EQ.