The device linked to was just an example piece, here are more
, and another
. The concept is that you would use one of these to take speaker cable, from your "B speakers" output on your receiver, plug the speaker cable into the converter, and then the RCA connections from converter to your wireless transmitter. Then select on your receiver both speakers A+B. Speaker A are your speakers, and in this case speaker B will be the sub. This is the only (easy) way to be able to control the volume of the speakers and sub together with your particular models of receiver and sub, since the receiver doesn't have any preouts/LFE out and your sub doesn't have speaker level inputs. You would only need to adjust the volume on the sub amp during set up, and then whenever you change the volume on the receiver both the speakers and sub will change their volume together. (We'll call this option A) This does require the B speaker outputs on your receiver, if you are running two pairs of speakers from this receiver as it is, then this will not be possible. If that is the case, you'd need to either buy another receiver or use the sub in your main system (sorry if that comes across as rude, its not meant to, just being practical).
In regards to the tape out question, as I mentioned above, technically it can work. The problem is that the tape deck output is not volume controlled. So if you decide to use that option, you will not be able to control the volume of the sub through the receiver (the receiver will always be sending full volume signals out of the tape deck connections). So if you want to change the volume, you would need to change the volume on the receiver for the speakers, then walk to the sub and change the volume for the sub through its own amp (we'll call this option B).
Technically this (option B) would work, but to me it seems it would be far more of a day-to-day hassle then buying an inexpensive line converter and using that (as in option A).