How Does a "HI-LEVEL IN" Affect Impedance Load?

I'm looking at adding a sub (possibly a used Sunfire MK2) to a 2-Channel integrated tube amp, with no pre-amp/sub outputs. Would it be ill-advised to connect (in parallel) the speaker outputs with the Hi-Level In, on the sub? What effect. if any, does this have on the impedance load? The currently-used (custom built) speakers run around 6-ohm, and the amplifier (Jolida) does have both 4, and 8-ohm terminals, for whatever that information is worth.  

Thanks!

-Brian