Use the Direct Output . On the rear of each channel, there is a TRS jack labeled "DIR OUT." This signal is post-gain, pre-EQ, pre-fader by default. To make it post-fader, move the internal jumper (JP1) inside the channel module. Do not use the Insert jack for this.
Over 20 years, the M-2600 MKII user community has reverse-engineered tricks that Tascam didn't print. While the manual won't tell you this, consider these "hacks": tascam m-2600 mkii manual
The last 15 pages of the manual are pure gold for technicians. You get: Use the Direct Output