How distorted with sub line in or speaker output

I thought about adding a sub, to my integrated amp...Thought about getting a sub and connecting the amp through the speaker line ins and then run speaker cables out to the speaker. The advisor at a store told me that the best way of connecting a powered sub would be to do it First and foremost through an RCA line in from a preout, Second through a speaker line in. He said that the sound was going to be fine, but not excellent if I chose to do it through the speaker line in.
The integrated I have is a 47 Labs Shigaraki integrated, which is known to provide an extremely pristine quality power output. The sub in question was a Infinity Intermezzo 2.1? The one with a 1000 watt class D Bash amp.
Any opinions, I would like to hear from.
I always use RCA connections from preouts to sub.....not only is it simplier/cleaner installation, but who wants the extra cost of another set of speaker cables plus the extra connections to degrade sound of main speakers? Also since ICs used only to connect sub they can be cheaper cables than system ICs.

I use sub for 2 channel stereo and only to fill in bass below output of main speakers.....don't have any experience with Infinty subs, just general info given above.
I have a Bryston B-60 and originally connected my Velodyne CT100 sub by removing the pre-out/main in u-bars, running the rca's to the sub from the pre outs and then back from the sub to the main ins. One day I got to thinking..."how much distortion etc. is being added by putting the subwoofer electronics in the chain of electronics feeding my B&W CDM1nt's" I unhooked the IC's to the sub, put the u-bars back in and decided to run cheap speaker cable from the speaker jacks on the B60 directly to the sub, so now there was nothing "in between" the preamp & amp sections of the integrated. My first impression was WOW...this is what the Bryston actually sounds like through my B&W's. I never went back to the old way of hooking it up, and after reading a number of posts...some other far more knowledgable audiophiles seemed to have already answered this question in other peoples posts and concurred with my new way of setting it up. Your situation may be completely different as I am not familiar with your amp, but my feeling is...try it both ways & see what sounds solution may only be unique to my own setup.

just my 2c

Using the high level inputs is always gonna be better. If your amp has two sets of speaker outs, then its nice and easy. Using the preamp outputs gets the signal to the sub a little slower. And low frequency waves are already slow. I have change a few friends systems to speaker level inputs, and they have all been amazed at the difference.
Hi, I'm currently using an Audio Physics Luna sub with the Merlins and I have tried both ways in my system. I have found the integration to be better running parallel as opposed to rca inputs. For one thing, volume control will integrate more seamlessly, meaning you don't have to manually adjust the sub volume control everytime you change volume for your main speakers. Another thing, and this is more important, is what Hifidreams have already mentioned, you want the sub signal to travel at the same rate as your main speakers. Otherwise, you're going to have a heck of a time with the timing and positioning of the sub. I spent months trying to fix this problem with rca connection to no avail, and finally gave in to using the speaker line.
I have passed my veredict. I will purchase the subwoofer and run it through the speaker level inputs parallel to through the same speaking binding outputs. One set of speaker cables will go directly to my horn fullrange loudspeakers, while another speaker cable set will go directly into the subwoofer. This set up will prevent me adding one more low quality crossover component where it counts, in the frequencies between 50hz-18,000 hz. Thus, there will be parallel reproduction of some frequencies, as the full range speaker will go down to about 35 HZ, but when the occasion calls, the subwoofer will fill the frequency with more volume and I will not need to bother setting the volumes of the two components independently, just need to adjust the subwoofer output to my taste. Thanks for the input. Never thought about the slowness of the low frequency signals travelling parallel to both speaker components. Learn something new everyday.