The previous owner had an additional set of RCA connectors installed for running some sort of sub within his system. I've come to find out that these 2 connectors won't work with my JL Sub ....Can you provide further details on why this is so? I would expect that if the amplifier was modified to provide line-level outputs on RCA connectors for the previous owner's sub, those outputs should work with pretty much any other sub that provides line-level inputs.
Also, as a point of information, there is probably an additional reason why using the tape jacks would not work, besides the one Spinaker correctly cited. On the 801A version of the amp, at least, the pair of tape jacks that is provided are inputs, not outputs.
Regarding speaker-level to line-level converters, all of those I have ever seen are for lo-fi applications, such as in automobiles, and would not do justice to your JL sub. Also, many of them are not rated to be able to handle the voltages your amp may put out on its speaker connectors, without being damaged. It is possible to rig up a converter yourself, using resistors to divide down the signal level, but you may not be looking to undertake a do-it-yourself project.
So I would focus on what the problem is with the connections the previous owner added. Not sure what else to suggest.
Regards,
-- Al