The Formula No. 5 was inspired by old tweed amps, most notably the 5E3 circuit. The character of the 5E3 is very unique. It has very minimal power filtering, low plate voltages, elemental tone stack, and does next to nothing to control the low end between gain stages, not to mention the under-rated output transformer and speaker. The entire system runs inefficiently resulting in a loose bass, greasy mids, cutting treble, and incredible grit - none of these qualities were ideal or intentional back in the day. In spite of this, over the past five decades the amp inspired some of the coolest, most sought after guitar tones ever recorded.
Catalinbread has captured the unique qualities of these amps in the Formula No. 5. For this circuit they felt cascaded JFET gainstages yielded the best result. They have a softer sound than MOSFETs. JFETs also have much more natural attack/decay than diode clipped rigs which are pingy on the attack and fizzy on the decay. They chose capacitor types that enhanced the midrange and burnished the treble.