I'll start with Music Lover's question and work my way back. The classic 100s, which I own, are the closest in look to the Isobariks. Like Isobariks they are a big box on an open frame metal stand. They are about a couple of inches larger in each dimension so they don't appear to be that much bigger. However, since they sound the best out into the room about 18", as opposed to the close wall placement of the Bricks, they are more imposing in anything besides a large room. I think this photo gives you a good idea of the size and look:
Now onto the question of a sub with the ATCs. I was out of town for the holidays when the question (which was at least partially aimed at me) was asked or I would have answered sooner. So, lets start with connection. I have my ATCs connected to my KK through the unbalanced (RCA) outputs as I felt it sounded more musical than the balanced outs. This leaves me free to use the XLR for the subwoofer. As per Fredriks comment I have the sub, which is to the left of and behind my left speaker, connected only to the left XLR output from the KK. As I mentioned in my posts on adding the sub I found this was a touch more tuneful than having it connected to both channels output.
As to the setup, Fredrik's post covers the technique quite well with one exception related to REL subs. I do agree that you do not change the position of the main speakers when you add a sub as the speakers are positioned to where they interact most musically with the room. The sub needs to be set up the same way, starting from the corner as Fredrik mentioned. With REL subs you will also find that the rotation of the sub makes a difference with it sounding best aimed roughly diagonally across the room. The difference in the setup technique with the REL subs is that the crossover is tuned to the main speakers just like the level is so the tuning of those settings need to be done with the main speakers playing. The positioning can be done most easily with the mains disconnected although I am used to doing it with them connected as well and the result is the same either way in the setups I've done.
This brings us to the question of the value of the sub. I don't see the main job of a sub being adding low notes you otherwise wouldn't hear. There are really very few notes that are lower than what an ATC 50 will reproduce and even fewer below what the 100 puts out. The rolloff figures ATC give are pretty conservative and they are likely to go a bit lower in room. It certainly seems to me that my 100s, which are rated down to 32Hz, go as low as my Isobariks which I know were good to at least 25Hz. I find that a good sub gives an improvement in the quality of all music well up into the midrange giving a more natural and tuneful quality to vocals, guitars, pianos, violins, etc. as Christian noted. It also does give the music a more spacious quality giving more of the feel of the performance area. I certainly feel this is a worthwhile improvement.
Of course, whether it is a cost effective upgrade depends on the rest of your system. As it comes at the end of the playback part of the chain it certainly isn't the first place to upgrade a system. But if you have optimized or maxed out the source and control parts of your system as you and I have then I find a sub a worthwhile upgrade (although I'd put a Harmoni rack before a sub in the hierarchy as it improves the source and control quite noticeably). Again, I must comment that my finding of the improvements from subs is based solely on my experience with REL subs, the B1 being the one I use in my current setup. And if you are going to get a sub it is important to get one big enough to be able to keep up with the output levels of your main speakers. This has more to do with the overall design of the sub so you can't just go on driver size or amplifier power. I hope this is all helpful.