A receiver is basically a pre/pro, amplifiers and a tuner in one box. This give you the advantage of simplicity, and in the ever changing world of HT we are currently in also gives the most up to date units as separates take longer to develop and come to market (don't ask why).
Going to separates (pre/processor, amplifiers and a tuner) allows each component several advantages.
They each get their own power supplies (one of the most important and frequently overlooked areas in HT)
They each are more isolated from each other.
They each can be changed independently of the other parts.
As for more bass, it depends. Certainly with separates you can get more power, which generally will give more, and more importantly, more accurate bass (with the same speakers assuming you did increase power or went to a much better amp.....), but that is not all you get. You also get a lot more clarity and detail. This does come at a price. You need extra cables, possible more dedicated outlets (if you haven't got any dedicated lines to your HT, then once you buy the home, GET SOME!!! They are the best bang for the buck you will ever spend on your system!) and more rack space.
As far as you living in a town house and worrying about power, I used to live in a condo and had my front speakers actively powered with three 200 wpc dual mono amps, the center and surrounds each were powered by one of the same amps. Yes, it would crank, but that is not the point. You can get volume easily. Buy efficient speakers and something that powers them and you are there. The increase in power I had and the active crossovers made the system so amazingly clean and clear that all the headaches were worth it.
Now in my current room, this system didn't do well, the room is open on one and a half walls and the added room volume wasn't handled well even with all that power....
FF a few years, and now I am using less power by superior manufacturers and getting much more dynamic, clean and lively sound than from the prior system in a bigger room.
I guess what I am trying to say is it's complicated.
Poor separates could sound worse than the better receivers, but good separates can always sound better than receivers, though the receiver market is really working to break that, they aren't quite there yet.
Ken Taraszka, MD