You don't need a Direct Box for bass or electric guitar, on the Portastudio's Mic/Line inputs.
Just plug the bass & guitar directly into the 1/4" mic/line inputs, on channels 1-4 [mk2] or 1-6 [mk3], and with both the Master AND Channel Fader set to about "7", adjust the TRIM for a VU signal that peaks at about 0VU to +3VU, but never higher.
Unfortunately, that adjustment technique is by the book, and it doesn't get much better than that, however inaccurate it may be, because the 424 Series does not have OL {oveload} LED's, like several other previous models of Portastudios. An OL LED on each channel was more of a foolproof way of minimizing & eliminating preamp-stage overloading.
