Best way to integrate a sub into 2 channel system?

I have looked at many of the threads relating to this subject but was unable to come away with any firm conclusion. I have a tube preamp and amp and bookshelf speakers plus an active sub. Please tell me if one of the following methods is considered best:
1) Use the high level connections(speaker wire) from the amp and then to the speakers using the sub's crossover

2) Use the low level connections (RCA) from my preamp to sub to amp using my sub's crossover

3) Buy an eternal crossover (Outlaw ICBM) and place it between my pream and amp to send the low freqs to the sub (low level connection/RCA) and high freqs to the amp. I know that I need to disable my subs crossover in this case.

With 2 and 3, I know that I would be relieving my amp of having to deal with the lower bass freqs and potentially improve the mids and highs but run the risk of the crossover causing problems.

thanks for the advice,

Chris