My concerns are:
1. timing issues with adding another speaker affecting imaging/soundstage etc.
Not a problem. Human beings hear low bass frequencies so much differently than mid-bass and up that its simply not a concern at all. Subs can be placed anywhere in the room and unless crossed over too high you will never be able to localize them.
Please note: subs. Plural. One sub is only slightly better than no sub. One sub inevitably results in one set of big room modes, ie lumpy slow bass. Multiple subs, 4 is recommended, results in less sub having less output producing more smaller modes for much smoother more articulate bass.
Multiple subs is the secret to really deep, articulate bass. The kind of bass that accurately recreates the low frequencies of a large space. This kind of bass actually improves imaging.
2. Remote volume control on the sub since different songs may need different amount of sub?
Well are you after accurate bass? Or a tone control?
A proper distributed bass array with truly smooth, accurate and articulate bass will be there when the music calls for it, and unnoticed when it does not. If you're looking for advice on how to get a lot of boomy bass whenever you think it should be there look elsewhere. Not my bag.
I can place the sub in any area of the room.
Excellent. Except remember its subs, not sub.
Best placement usually will be asymmetrically distributed around the room. But if starting with just one be aware you will have to move it around a lot trying to find the least bad location.
I also read on this forum somewhere about the pros and cons of connecting subwoofer to speaker outputs, but can't seem to find the thread, nor do I recall it ending with an answer if it can potentially cause amplifier problems or not.
This ones easy. If your sub accepts speaker level inputs then use em. If not then the easiest solution is what's called a Line Out Converter or LOC. Quality does not matter, except for appearance. So buy cheap, parts-express.com has about the best, by which I mean cheapest, because quality does not matter! Or if you are comfortable soldering, or even if you just want to know what the heck is going on, this is the single best page on the web
http://www.epanorama.net/circuits/speaker_to_line.htmlWith this circuit and the $2 worth of resistors it calls for I was able to convert my tube integrated to add a pre-amp out that I have been using to run my two Dayton amps powering my distributed bass array for nearly a year now. So you can be sure all the info above is accurate and based on actual experience. As opposed to say what you may have "read on this forum somewhere".