Yes, that should work, assuming the line stage preamp provides sufficient gain (which I think most would).
The only other concern which occurs to me is that sometimes hooking a tv system into an audio system, especially if the tv is fed via cable, can lead to ground loop/hum problems. But if that occurs, you should be able to resolve it via some combination of an in-line ground isolator for the cable, rearranging your ac connections, cheater plugs, etc. And perhaps your results going directly into the passive preamp are already indicative that you won't have that kind of problem.
Regards,
-- Al
The only other concern which occurs to me is that sometimes hooking a tv system into an audio system, especially if the tv is fed via cable, can lead to ground loop/hum problems. But if that occurs, you should be able to resolve it via some combination of an in-line ground isolator for the cable, rearranging your ac connections, cheater plugs, etc. And perhaps your results going directly into the passive preamp are already indicative that you won't have that kind of problem.
Regards,
-- Al