I think once it reached a certain volume, the level would be too much for the line in on the sub pac - if you cranked the volume up to say, 1 watt output to power the abyss, I'm sure that's much more than the sub pac wants to receive for a signal.
I think with the portable players, the maximum output power is around line level anyway, so it doesn't really matter but the vio packs a much bigger punch than that. I have a aux cable to casette tape thing to plug my ipod into the work van's tape player and if I turn the volume right up on the ipod, it gets really bloated and boomy and cuts out and all sorts of stuff because I guess it's just more signal than the tape head is meant to receive. It's happy around 2/3 volume.
I may be wrong - someone feel free to correct me, but I'd use the vio's line out if it were me