Subwoofer w. Passive Line Stage?

I'm using the Placette passive volume control to drive a Mac Power amp. If I want to add a powered subwoofer to my main 2 speakers, is there a way to do so by only tapping into the power amp's output at the speaker terminals? The Placette has only one line out and lacks the current to drive two amp inputs.

Some powered subs are set up for this. I'm pretty sure REL is and Audio Physic may be also. Mnay subs ae not set up this way, though, and I would like to hear if there is a way, maybe transformers, of converting speaker level outputs to line level.
You can go into the sub and out to the amp using the sub's crossover. Or if your sub is equipped, you can go out of the speaker outs on the amp into the sub and on to your speakers. The first way is the best.
Vandersteen 2w and 2wq drive off of the amps output, but they do place a passive filter (also known as crossover) between the pre and the amp. I do not know that would interact with the passive, but they are fantastic subwoofers.
Thank you for your help. My dilemna is this: I have an extremely high quality (and expensive) connection straight from CD/SACD through Placette passive potentiometer through to power amp, and I don't want to pollute the signal by passing it through additional cables and crossovers. So I like the idea of simply tapping off the amplifier output into a transformer circuit built into or near the subwoofer that would convert the high-level signal to a line-level input. That way I save tons of money and preserve high quality signals for my main speakers.