I can’t buy another component, I have to make what I have work.
I gotcha; I totally understand.
But I am going to ask anyway if I am understanding this correctly:
Just one RCA out from preamp.
You are saying just one pair of RCAs (L&R) out from the preamp (going to the amp), correct?
That's what I interpreted, anyway, when you said you had no sub out in your preamp. I understand what you were saying about not buying another component, but I'll clarify what I was saying anyway:
the highpass filter goes between the pre and the amp. In other words, that one pair of RCAs out to the amp now goes IN to the highpass filter. (Highpaass filter then rolls off the bass, but I am not sure at what frequency). There are two pairs of RCA outs in the highpass filter, and the pair with the bass connects to the subs (although in my case it was sub as in singular), and then the pair with the remaining treble goes to the amp.