acmestone:1a) Could this cause anything dangerous? I don't know what internal crossover can do if I connected the inputs: 2 grounds (probably fine) and 4 signals (issues?).
I wouldn't say dangerous, just may affect sound quality... i might be out on a limb. Give it a shot and see what happens. I've never done it myself, I've always had everything connected.
acmestone: I've seen mono-out to stereo-in before, and I haven't seen any negatives, but this BASH subwoofer can carry enough oomph to make me worry if I connected the 4 channels together to read the single bass input.
To be safe try the one, and see how one goes first, need more bass, add in second, third, fourth.
acmestone:Also, the receiver I'm using can optionally (if no LFE) output "LFE + MAIN", which is the "bass elements of the front speakers' channels," so it's not necessarily just dolby digital/dts LFE.
Your trying to use the systems as a dedicated subwoofer. It normally expects the full range singal from each channel. You can try the LFE+MAIN or just LFE. Experiment. Start at low volume and go up slowly, see if you hear any sonic differences.
acmestone:1b) I think the cleanest way would be to wire the LFE directly into the un-amplified subwoofer input inside the box itself, if I could. I have no clue if the satellites are amplified on the same path as the subwoofer, and I don't know if I can get a circuit diagram, but I'd like to avoid the satellite circuitry altogether (since I'm going to be opening it up anyways). Any idea if this is possible?
Are you comfortable opening up and working on the amplifer? I doubt your going to find a schematic, so it would be interesting if you know what you where looking for in terms of tapping into, but I bet it would be tricky. I'm guessing the system uses a crossover to feed the signal to the sub, so you'd want to tap in after that, but what the exact operating range is would be unknown. I would expect the sub and sat amps to be after the crossover.
acmestone:2a) Center speaker: I've looked for a PM5.1 center speaker, but none have popped up on eBay yet. I've seen some other nice Klipsch ones on Amazon and eBay, like SC-1, SC.5, and C-1, within my price range. Would these go well in my setup as a new center speaker?
I would think even a Promedia 2.1 satellite would be a closer match sonically, plus cheaper, I see a pair for $25, so you can just use one.
The last thing I can think of is that your still going to need use the control pod. Just to note.
The STIG's fat cousin.