HZ.....How low for full range music???

The lowest fundamental tone a string bass plays is at 41hz per second.

The treatment of overtones is not relevant to your question but remember the difference tones. A difference tone could be called an "undertone". A fundadmental of 41 will generate a difference tone of 20.5,which will generate a difference tone of 10.25 and so forth. These later tones are felt as much as heard and are responsible for the visceral ambience in a live performace that is(for me) outrageously expensive to recreate.

A poster above mentions an elaborate array of subwoofers and based upon the quality of his posts,I believe he has recreated that ambience.

I'm not a subwoofer guy(which is my problem,of course) and am happy if enough of the fundamentalsin the forties and fifties are present to represent the harmonies involved.

Hope your friend finds speakers he likes.

One issue that never gets mentioned is the SPL (loudness) at which the LF extension is measured. At very low volume I can get close to 20 Hz with some small Dynaudio speakers which I have. But, as you turn the volume up, the bass volume stays the same, so that at normal listening level the 20 Hz capability is for all practical purposes, zero.

IMHO, frequency response should be spec'd at some SPL. Perhaps the same level used for the sensitivity spec.

You may want to conduct a simple test. Go to an audio salon, and listen to whichever speakers you like, both with and without a subwoofer. You may notice quite a discernable difference when the subwoofer plays.
I heard a REL subwoofer in such a demonstration, and was amazed at how much "fuller" even quiet passages with vocals sounded with the sub!
I also owned Maggie 1.6's for a while, and was content with their bass, for a while. However, after adding a Vandersteen sub, found that it improved the entire listening experience dramatically. It's strange, but having those lower frequencies seems to take some of the edge off the highs...
It was recommended to me by an owner of a high end audio store that I should run a pair of subs, since there's plenty of low freq. information sent to each channel. I ended up buying his own pair of Vandersteen 2W subs. It's true, especially if listinging to such music as smooth jazz, synthesized, etc., two subs do make a stereo image of the low end. With one sub, you hear the boom, with two, you hear which side the boom is coming from!
Now I'm running tiwn pairs of Eminent Technology LFT-8a's with tiwn Vandersteen subs. That's a total of 10 eight inch drivers for bass. It's incredible! Powerful, precise bass, without distortion. It doesn't have to play loud to sound authoritative.
Once you familiarize yourself with good, clean lower frequencies, you'll know when you're missing it listening to other speakers/systems.
Two times I have tried monitors with subs, and both times I went back to floor standing full range speakers. If you want good bass representation in your system, do full range floor standers, and subs. But as previous posters have said, better quality with a couple less hz than boomy boxes with distortion.
You may want to read reviews of speakers on audioreview.com

Some good affordable speakers I have used with clean bass are Vandersteens, Magnepans, Eminent Technology (I switched from Vandys, to Maggies to Eminents).
For economy you can even go with a setup like a pair of Wharfedale Diamond 8.3's coupled with a sub. I had a pair of them in my system and was amazed how good they sounded for an economical speaker. Great bass. Biwire them or biamp them and you'll have nice sound. It's all how many $ you want to throw at the issue.

Here is a website with information that may be interesting to you. It is from TNT and describes instruments and their frequency range, as well as their volume.

http://www.tnt-audio.com/topics/frequency_e.html

It is surprising to most that the concert piano goes down to 27.5 Hz. Of course, there is synthesized sound, organ, bass drum, and even cannons (1812 Overture)!

If you believe the tests that sub manufacturers have done, there is subsonic information that adds to the perception of sound. Plus, everyone has different sensitivity and abilities to perceive sound frequencies. I believe that most reproduced music in our homes needs a subwoofer to deliver all the information. I use a Hsu VTF-3R Mark II below 50 Hz underneath my Dynaudio Audience 70s (measured at -3 dB at 34 Hz), and, in another room, an REL Q108E under my Von Schweikert VR-1s. Good luck!

Not many speakers are able to do justice to frequecy around 30 Hz or so and those that do are not cheap at all. I also owned Maggies at one time and I think that they are better capable of reproducing bass than most box speakers at similar price level (cleaner). Lowest note which a piano can handle is 27.5 but is rearely heard in any music CD. Next up the range is the double bass which goes down to 41.5 Hz on its own.

I would choose high quality speakers (the best one can afford) and stick to its lowest frequency reproduction capability at/around 40 Hz or so. A speaker like this would probabaly take care of 99 percent of the albums in general.

If more bass extention is needed, a sealed subwoofer like Rel Strata will come in handy, which is extremly versatile in coping with real world room acoustics.

All the above is just my own personal opinion, based on the various equipment that I owned over the years.