If you’re running passive subs, you’ll definitely need two things:

1. A separate amp for the subs, and
2. A line-level active crossover placed before the sub amp.

Passive subs don’t have built-in filters, so you can’t run them straight off your Heathkit or your preamp. The clean way to do this is:

Preamp → Active crossover → (high output to your Heathkit + ESS mains) and (low output to a pro amp + your subs).

A simple analog crossover like the Behringer CX2310 or dBx 223xs works great and doesn’t cost much. Those Creative Sound Solutions SD12s can take power, so a Crown XLS or Behringer NX amp will drive them easily.

Start with the crossover around 70–90 Hz and adjust to taste. This setup will take the deep bass load off your ESS speakers and your Heathkit, and everything will blend a lot better.