One sub, a two channel system ,one HT reciever ??

It is reasonably simple. When you use the line level in and out you are using the crossover and rolling your main speakers off at the subs crossover point. When you use the speaker outputs you are NOT rolling off the main speakers. In theory the first has the advantage of lessening the load on the main speakers because they no longer have to reproduce low bass. Nevertheless, I find the second method to work much better. I currently have three types of subs [REL, Nelson Reed , and HSU] and have had others in the past. The critical factor is not having to run your signal through the crossover. I have yet to see a crossover that did not degrade the sound. I am assuming that in the first case you are running the pre out to the sub input and the sub output to the power amp in and thus using the subs crossover. Since you did not name your equipment I can't be more specific. If this is not true advise me and I will post further. When hooking up equipment I have always found it helpful to visualize your system as a stream with your signal source at its head. Just follow the stream toward your speakers, hooking it up as you go.

The idea behind not running the signal through a crossover is that it represents an additional degradation of the signal before it has been amplified, a degradation here can never be recovered. The crossover inside speakers represent a division of frequencies of an ALREADY AMPLIFIED signal and has far less effect on the sound. I am unfamiliar with multichannel systems and so don't understand the purpose of setting the crossover at 120hz but in operation no sub should EVER be set this high. Anything operating in that range is not a sub but a woofer. I run my subs as low as possible, I never have the sub crossover set above 40hz. In this way the output of the sub is nondirectional and it supplements the bass of the main speakers instead of interfering with it. Again I am writing from the perspective of 2 channel sound and of someone who uses subs with response below 20hz. The Nelson Reed was specified to do 120db at 20hz and extend to 16hz. The REL and HSU will go almost as low but not nearly as loud; they don't have 8 12" Volt drivers as the Nelson Reed.

I am currently running Spendor SP-1/2s with a REL stadium 11 sub. I am using the speaker level inputs to the sub rather than the line level which would allow me to roll off the bass. Why. First, this is what REL recommends. Second , I have never had an active crossover I couldn't hear. Such may exist but they would be very expensive. A custom passive crossover would be the way I would go if I was inclined to use one. For me the critical factor it that running the main speakers full range is what they were designed for. Rolling off their bass may enable them to play louder but when I have tried it I found the benefits mostly theoretical and the drawbacks obvious. BUT, if I have learned anything in 45 wears in audio it is we all hear differently and listen to different things in music so it a personal choice.

Bob , without getting personal I would like to ask if you have ever heard ANY of the equipment I have described. I am describing experience, you are mouthing platitudes The Spendor , for example , was designed by the BBC and used around the world by other national radio networks as monitor speakers. The Nelson Reeds were designed as monitor speakers for movie studios. I probably made a mistake posting on an AV Question as the performance parameters appear to quite different. I was a Quad dealer for over 20 years and of course they are famous for distorted sound. Note to self, no more AV postings.