Incorporating sub into my 2 channel / HT system

Iv'e been reading many blogs but still can't find exact answer to my question. I have a Marantz SR-18 receiver that I use for my HT. This only runs my center channel and sub......I don't use surround speakers as it's only a 3.1 setup.

I use a Cary CAI-1 integrated for my 2 channel listening. The Cary is also used to run my L/R mains during HT via cinema bypass loop. My mains are JM Lab Electra 926's my sub is a Velodyne SPL-800R which is currently run off my Marantz using the LFE. I want to augment my 926's during 2 channel listening as well. How do I connect my sub so that it performs double duty weather 2 channel listening or HT. Also, I want to add another SPL-800 to augment both L and R sides. I am a babe-in-the-woods when it comes to this stuff. How do I tie everything together? which in/out do I use? Do I need Y-splitters? Help!
Does you sub have separate inputs for "LFE" (from an AVR or receiver sub output) and Line Level inputs for Left and Right?

And does your Cary integrated amp have Left/Right preouts?

If so, then run a sub cable from your Marantz to the LFE input on your sub and run 2 sub cables from the L/R preouts on the Cary to the Left/Right inputs on your sub.

When you watch movies, the Marantz will send LFE signals to your sub and the Cary will be in HT Bypass (so the preamp section is bypassed and it is nothing more than a power amp in this mode, so no bass signals will be coming out of the preouts). Your Marantz will apply any bass management to the sub.

When you listen to music, turn off the Marantz (or mute it), and select your source device to listen to and the Cary will send signals via the preouts to your sub. You will have to 'manually' adjust the sub's high cut off to get the best integration of sub and mains as you can.
Unfortunately, your Cary CAI-1 lacks preamp line-level outputs, so you have only one choice.

Connect the second set of speaker output terminals of the Cary CAI-1 to a switch (DPDT on-off) and that switch to the the sub's speaker-level input terminals. Leave the sub out from the AVR connected as-is.

To use this setup for HT, make sure the switch is open (sub not connected).

To use this setup for 2 channel, make sure the AVR is OFF and then close the switch.

The problem is that the level/crossover in the sub must be the same for both operations and that will never be ideal for both. There is no reasonable solution for this but I cannot think of any other way with the equipment you have.
your sub doesn't accept mutliple inputs, most don't. I will only run from the LFE, line level, or speaker level connections, individually, but not process signals from more than one input at a time.

The only subs that I know of that will take multiple inputs are Definitive Technology or Rel. Both of which make great subs.

I would suggest multiple Rel T1's or Def Tech Supercube I's, running from 2 channel and LFE.
Several subs have multiple inputs.

Polk, eD, Velodyne, SVS, Epik...list goes on.

I have my Epik Valor sub hooked up to both my AVR (LFE input) and Musical Fidelity A5 integrated amp w/ HT Bypass(Line line L/R) in my hybrid HT/2-channel rig.

No more cable swapping on the sub, no switches needed.