If you are looking for a really good system then, as was mentioned, it's a matter of balance. It's going to cost a lot of money to get speakers that can go below 40HZ with good accuracy. One critical item is to look at the frequency response that the speaker can manage within 3db. If a speaker system says, for example that it has a frequency response of 30-20000HZ but DOESN'T state a db figure, don't even look at it. Any speaker can "twitch" at 30HZ, it doesn't mean you will hear any sound. So a good speaker should show a freq range response curve with a db figure. 50 to 18000HZ +/- 3db is quite good and way beyond what most mass market home systems can do. Any speaker system that can accurately get into the 30HZ range is superb and you will pay a lot for that ability.
The most important thing is smoothness. You can add a good 12" subwoofer to any system and get bass down to around 20HZ but unless it is accurately integrated with the system, the result is most often a horribly mismatched response curve and a horrible sound to match.
The speaker freq response curve is ideally a straight line from bottom to top - 20-20000hz +/- 0 db. This isn't possible but it's the goal. Soundwise it's better to have a system restricted in the bass and treble but with a "straight line" response curve in its usuable range than an extended range system whose response curve goes all over the place.
There are a lot of respected brands in the speaker industry and usually within those brands, more money buys you a smooth accurate extension on the bass and treble end. There are many brands but two that have been around for a while, have an excellent reputation with a decent range in price include Thiel and Vandersteen. No, I don't own either one.
I'd suggest that you have to spend at least $1500 to get a decent NEW set of speakers of good quality. They go up and Up and UP and UP from there.
However, there are obviously used speaker options. One "old" speaker that was basically the standard in the 60's and early 70's was the Acoustic Research AR3a. They are enjoying a resurgence in the used market and are not cheap for an old speaker but they still sound impressive and their bass capability shame many current "audiophile" speakers. You can expect to pay 400+ for a pair in good condition. And, like our old Hatts, parts are available; all the drivers, crossovers and controls for them can be purchased. AR LSTs are an even more capable and fairly rare "old" speaker. They easily fetch $1500 in good shape; they are essentially a three-panel AR3a with 2 additional midrange and 2 additional high range drivers for more power handling and better high freq dispersion. The LSTs will embarass ANY current speaker made selling for less than $5000 and many above that.