You could use the tape out as a last resort. Don't use the Phono.
Does your Sub have High level inputs? That is Speaker level Right and left inputs and outputs? That would be a better way to go if your sub is equiped with high level inputs. If so just connect the FRONT left and right speaker outs to the left and right speaker ins and connect your Speakers to the outs.
Tape will work but be sure to use the Right or left line level input (NOT the LFE input) That will allow you to use the subs Build in Crossover. (if it has that)
If you use tape out to the sub you won't have any volume control for the sub. Unless you have pre out connections on your receiver any low level output you use will have a fixed output level and will not control the volume of the sub. If that's the case the only input on the sub to use would be the high level ( speaker) inputs on the sub.
If you don't have a Subwoofer Output on your Receiver there is no way to balance the subwoofer DB's to your Speaker level without running the Fronts through the Sub.
If you would go the Tape out route the sub will always be receiving the same input db's, thus when listening at lower volumes the sub will not be at the same consistant level as your front and surround speakers and your sub will not increase or decrease with the amps volume control.