I've had plenty of hands-on with hiding stuff, and have installed Stealth Acoustics products.
When looking at making your speakers disappear, you have to weigh several factors that work in opposite directions. The degree of invisibility vs sound quality for one, placement for another. Stealth products are very good, but the serious audiophile won't like them much. So you have to ask, how important is it for speakers to be completely invisible vs sound great? Frankly the Stealth products are a good trade off, leaning obviously toward invisibility, but with sound entirely acceptable for decent background music and basic HT. There are now other players in the completely invisible realm.
The Artcoustics products are an interesting combination of a piece of artwork hiding a speaker. Again, performance is good, not flat-out audiophile, but a bit better than the Stealth. However, in a old home, a modern bit of art may not work well. You can custom-print your own Artcoustics panels, of course. There's a bit of conflict in placement also, where art looks good, speakers may not work the best. Again, it's a compromise, often a very livable one.
Then there's the option of using more conventional in-walls and placing "art" over them. Custom-printed acoustically transparent fabric panels work pretty well.
As to wiring, man I feel your pain, but two-wire power circuits are not really that big a deal, as long as you have at least some form of ground, like conduit. If the wiring isn't shedding insulation, you probably don't have to worry about it. Depending on when the house was built you may want to watch out for aluminum wire, though. There was a copper shortage for a while, and some homes were built with aluminum wire. I think most have burned down by now. If you are in the plaster/lath era, you probably have conduit too, which makes pulling in new wire actually not too bad. It's a bit of a project, and mostly you have to figure out where things go first, and pull in new stuff a bit at a time, or hire an expert.
Subwoofers don't hide well. The article cited at the top of this thread completely wimps out on what to do about it. Hiding a big black box is just not easy to do. However, there are a few really good in-wall subs now, particularly those from Definitive, Boston Acoustics and Sunfire. They end up flush to a wall, yet move enough air to convince most people they are actually big black boxes. Most in-wall subs are passive, so there'll be a sub amp by your AVR to push it. I've also buried subs under floors and behind walls, then used a smaller opening in the room to let it play through, then covered the opening with a standard heating register. That's a pretty good compromise, and Boston Acoustics used to make an in-floor sub just for the purpose (I think it's discontinued now). The problem with this can be overly punchy bass, but a bit of calibration can take care of most of that.
Don't forget about some of the new wireless solutions, particularly those using the WiSA technology. If you got power for a powered speaker, but no audio wiring, it's a good fix, and WiSA works really well. And, when fishing wires in walls remember that older homes without insulation usually have fire-breaks, horizontal 2x4 blocks, at intervals between studs to prevent the stud channel from becoming a chimney in event of fire. If you run into one, get yourself an "installer's bit", a 48" flexible shaft drill bit, and drill right on through them.
