trying to add a subwoofer to a Mac system from before subs were a thing….

A Rolls SX95 will work well. I used an RCA Y cable to get the left channel input into the SX95. You will need a separate amp for your passive subwoofer. Inexpensive class D amps work great for subs. 

Solid advice!! American made, simple and like you say, place between the pre and the amp, so I'm only amplifying the freqs I want. Thank you.

I Do have a separate amp. The mighty McIntosh MC7300. I have two of these, one on the main, and one bridged parallel for the sub.

Anybody else beat this guy? If not, the Rolls it is.

DBX made a downward firing 15", subwoofer using a niobium, rare earth magnet.

"I think it was a, DBX model, (DB-15)".

 It came with a separate amplifier and uses the "Speaker Level, Line-output" from your amp to the Rt./Lt., as the inputs are marked on the DBX amplifier.

The amp states (150 watts). But I swear it hits like my "Martin Logan Descent" subs do with their huge plate amps!

So, with the DBX, you use speaker cable from the speaker outputs on an "integrated", (Usually called the "B Speaker" outputs). Or the same, "speaker cabling" from your component, amplifiers binding posts to the amp.

The (Rt. and Lt.) speaker cables are both used from your amp and then into the "DBX" amp. Which has an analog "Output Level" adjustment pot/dial, on the front of the amp as well.

I do still have it, in case you or someone else happens to be interested...

And as a for the price? "But for a song and a dance"! The MSRP was about ($800). I believe "From Crutchfield", purchased new back in the mid 1990's.

This is not a direct answer to your question, but I'd offer what I have done with a vintage Mac system and a newly added REL S/510 subwoofer.  The results have transformed my system with a new dimension of depth, and no hind of artificial or boomy bass.  The subwoofer simply disappears, but the added depth and spacial sense is transformative.  I have a Mc 2125 amp, C28 pre amp, XR 5 Mac speakers (along with the MQ 101 Environmental Equalizer and Speaker Control Relay Box).

The Rel S/510 is wired directly to my main speaker terminals on the Speaker Control Relay Box using REL's "standard high level connection cable."  The ground wire from the Rel connection is attached to the "c" terminal of my main speaker left output on my 2125 Amp (the right would work as well) to prevent a very nasty hum.  The Rel's are then adjusted for gain and Crossover frequency.  It was very easy to set up.  Hence in my system the Rel subwoofer is connected as "part of" my main speaker and receives the same signal from the pre amp, amp and speaker control relay box.  Trying to use the remote speaker option for the sub, and running it as essentially an entirely separate speaker, seems to me to complicate things and perhaps would make it more difficult to dial the sub in correctly.  I may be entirely wrong on this.  Anyway, hope this helps in a small way.