B_limo, it's hard to say which approach would be better in your case. I suspect that either one would work reasonably well.
I assume you are referring to the Asgard 2, as the earlier version does not appear to have provided pre-outs. Although I suspect that the <2 ohm output impedance spec that is provided for the Asgard 2 applies just to the headphone output, not to the pre-outs. But I would expect that the pre-outs on that unit probably also have fairly low output impedance, and the input impedance of the Classe CA-150 amplifier listed in your system description (assuming you are still using it) is a high 75K. So even if the input impedance of the sub's line level inputs is fairly low, as is often the case, the Asgard 2 most likely would have no trouble driving both components. And hopefully the cables you would be using to connect the sub and the Classe to the Asgard if you went with the line-level approach would not be unusually long.
In making those comments about the line-level approach I'm assuming, of course, either that the sub provides line-level inputs for two channels, or that you would be using two subs. If the sub also provides speaker-level inputs for two channels, or if you would be using two subs which provide speaker-level inputs, connecting at speaker level should also work well. And some would say connecting at speaker level is preferable because the sonic character of the amplifier would be reflected in the signals received by the sub as well as the main speakers. Some would also say that connecting at speaker level is preferable because it avoids the use of a splitter, but see the comments in my previous post relating to that.
Also, although it's probably not applicable in your case, be aware that special considerations come into play when connecting at speaker-level if the amplifier is bridged, or has balanced outputs, or is a pair of monoblocks. And as you've seen earlier in this thread, in the case of certain class D amplifiers.
Regarding the choice between connection of the sub to the amp outputs or to the speaker terminals, it may not make much difference but my instinct would be to connect to the amp outputs. Conceivably that could be beneficial by reducing the amount of "back-emf" generated by the speaker drivers that could find its way into the sub. On the other hand, though, it is conceivable that doing so might increase the amount of electrical noise generated by the sub's amplifier that would couple into the feedback loop of the main amplifier, assuming it has a feedback loop, so who knows? As I say, it may not make any difference either way.
So I suspect that both a line-level approach and a speaker-level approach would work in a reasonable manner in your case. My instinct, especially if I am correct in assuming that the amp is not bridged, balanced, monoblock, or class D, would be to go with the speaker-level method, connecting to the amp output terminals. But when and if practicable consider trying the splitter approach as well, and comparing results.
Best regards,
-- Al
