You should not do that. Speaker level voltage can be much higher than line-level audio voltage, and the speaker outputs are expecting to see an impedance load in the 10 ohm range, while the line-level input is probably in the 10,000 ohm range.
Subwoofer Connection: Speaker Level Amp to Line Level on Sub?
Hey there. Are there any real problems inherent in running a signal to a sub (SVS SB-3000) from an integrated (PrimaLuna EVO 100) - from speaker level on the amp to line in on the sub using an adapter? Thanks!