Sonicaps are a step up in transparency and clarity, adding a Miflex copper bypass would be a bigger jump, adding a little bit more richness and body to the treble.
The biggest benefits will be from swapping the two electrolytic caps in the woofer circuit for some good, budget polycaps. Upgrading the iron core inductor would also help. Last would be swapping out the resistors.
Gen 1. Gen 2 Sonicaps are just bypass caps, Most are under 0.1uF.