I had the same problem as you. I have a 47 Labs Shigaraki integrated, without any variable RCA output, like preamp outs and I had them running a pair of horn loaded speakers, but because I felt that the low end was not sufficient, I pondered about the question.
Some people will say that RCA outs are best, but when I posted the same question to some knowledgable people out there, they stated that the best method is to wire your the speaker outputs directly into an active subwoofer equipped with SPEAKER LINE INPUT...you will be running a pair of speakers cables from the same binding output or if you have another pair of free speaker binding outputs of the integrated into the sub.
The advantages of these are that for one, your sub WILL change volumes at the same rate as you increase or decrease your volume. Another is that because you run the cables from the same output, the speaker output, the sound will almost travel at the same rate to the subwoofer and also the speaker you have. Coherence and timing will not be a problem, as if would be if you have run the cable through a preamp circuitry, as in you integrated.
I disagree with the recommendation of DWELLER, for if you think about it, the original signal will begin to split from the CD output phase....while one signal travels to the sub, the other will have to travel to the integrated, to the preamp section then to the amp section, then to the speaker. Unless the amp section of the subwoofer has the same length in design as your integrated, this will not be optimal, timing wise.
I bought a powered sub and connected the way I stated. Two pair from the same binder....have not have problems and the sub has integrated flawlessly with the horns, and I did not sacrifice the sound signature of the integrated to gain more low end.
PAUL K
