In your original post you said 'Sad thing is that the sub has only line-in',
In your last post you indicate that it might also have speaker level in/out too when you say 'But then again while using the speaker-level input'.
If your sub has speaker level connections, that looks like the way to proceed.
If you don't have speaker level connections, it's just occured to me that you might be able to use your headphone output to feed the subs, but I'm not at all sure about what impedance and other problems you might encounter doing this. Don't rush out and buy a cable yet until other people have had a chance to warm you off this route.