What is the best way to hook up a subwoofer

Al, you are 100% correct about the outputs. Odd the SP-9 has two tapes and only one main output. That means the SP-9 will drive the sub full bore out of the tape output unless there's some way to control the output gain from the tape output. Just thinking outloud, I wonder if the single main output can be split???

Bruce, I'd feel pretty certain that the tape outputs are not volume-controlled by the preamp, and therefore can't be used. If the sub doesn't provide speaker-level inputs, using y-adapters with the single pair of main out jacks would be fine, IMO, provided that three criteria are met:

1)The sub provides separate line-level inputs for the two channels, not just one input jack that is intended to receive a summed mono signal.

2)The paralleled input impedance of the sub and the main power amp is at least 20K, as you indicated.

3)The total capacitance of the cables to the sub and the main power amp is less than 1000 pf.

If the sub provides high passed line-level outputs, connecting it between the preamp and the main power amp would be another possibility to consider.

Best regards,
-- Al

Al and Tiffany, I touched indirectly on the point Al makes in criteria 1) above, to wit, "[t]he sub provides separate line-level inputs for the two channels, not just one input jack that is intended to receive a summed mono signal."

That is the precise problem I was dealing with in the other comments I posted about the issue of summing the left and right channels. In other words, assuming you can use a y-adapter to split the single main output, the leg going to the sub will of course be both left and right channels. If the sub only has one input jack, i.e., the sub only plays one channel, then simply plugging the "sub-leg" into the sub will short the signals. I suspect you would wind up having only mono coming out of the other leg going to the amp.

In my case, I also run only one sub which is fed a summed signal via a special made impedance buffer. The buffer (1) presents a high impedance load to the output main feeding the sub and (2) sums the signals without shorting the outputs of the preamp. Take a look at my posts.

After absorbing what Al and I are trying to get across, I suggest running the idea by Calvin at ARC to check that the idea has "legs" (pun intended).