Does using a powered subwoofer make better sound?

I have Vienna Acoustic Mozarts. Wonderful speakers but their one weakness is below 40 Hz. I don't want to "harm" the beautiful mid-range and highs they produce. Would a powered sub give me the extra kick in bass I'd like? If so which one to buy?

System Theta Miles / Pro Prime IIIa DAC
Audio Research LS 15 Preamp
BAT VK-60 amp
In short: yes, a powered sub -- if it's the right one -- could definitely enhance the performance of your system. There have been a lot of posts on this subject over the past 3-4 months. I contributed a long piece which summarized the results of Widescreen Review magazine's tests of some 50 subs -- you can look for the post in the Audiogon archives, using the phrases "Widescreen Review" and "subwoofers".

Also, look for posts from other regulars on this forum. My personal experience with powered subs relates mainly to my Vandersteen 2Wq subs, which I think are outstanding, but other members of the forum have made excellent posts about other brands/models that will be informative for you.
Based on your situation, I believe a REL subwoofer would be a good fit (specifically, I'd look for a subwoofer with speed). The model that you will need would depend upon your room dimension. There are some trade-offs when using a subwoofer but it is my personal opinion the benefits prevail, especially in dynamics and bass when a speaker needs a little help in those areas. You will need to do a little work to position the subwoofer in the right place and to dial-in the crossover.

There are many threads that you can refer to for more opinions concerning your question. Good luck!
I second the REL suggestion. I haven't heard a system yet that doesn't benefit from a REL.
I am third for REL. It brings new dimention to my Proac 1.5
Does any of the music you listen to contain bass information below 40Hz?
The REL is the best in my opinion if your goal is to fill in the bottom without affecting what your main speaker do. Get at least the Storm III. It is worth the extra $500 over the Stratus III. The Stratus III is also very fine it you can't afford the Storm III.

The best way to explain it, is to point out that all notes have some bass in them. This is why a concert grand piano sounds fuller, richer than a upright spinet piano. If they are both playing middle C (or any other note) you would think they would sound the same, but they don't. The large enclosure of the grand piano resonates fuller. This sound you hear encompases a larger frequency spectrum than the middle C being played. The same is true for all instruments. For that matter, all musical instruments would sound exactly the same if all you heard was the same middle C. It is the sound surrounding that note that makes all instruments sound different.

Your speakers don't have a problem playing the high frequencies; the tweeter does not need a big cabinet to resonate. The bass is always the problem. The sub fills in the lower part of that frequency spectrum.

This also explains why a large room has a sound different from a small room. You can feel it when you walk into a large room (or hall) more than hear it.

You will notice much more than bass by adding a musical audiophile sub. You will notice more "air" or "space" between instruments because you are hearing the full spectrum of sound. That grand piano will sound more real. So will flutes, oboes, violins, drums, guitars, etc.

I never hear much about M & K subs in this forum. It all seeems to revolve around REL. My understanding is M&K "invented" the sub and is used exclusively by George Lucas. Is REL more of an audio sub as opposed to M&K being known for HT?
I have an M&K MX-150 and love it, though all of this talk about REL has peaked my interest.
P.S. I have my M&K mated with Vienna Acoustics Haydns.
REL has two lines of subs. The ST subs (Stratus, Storm, Stadium, Stentor, Studio) were designed with the Audiophile in mind. They also do a fine job for movies, if pure bass is your goal. The other line is the Q subs (Q100E, Q150E, Q201E, etc.) These are their HT line. They are also quite musical for the times you want tunes instead of movies.
Twl ... there are very few of my CDs which DON'T have information below 40Hz ... it may not be pure notes (e.g. organ pipes) it may simply be dynamics (e.g. kick drum, tympani). Furthermore I doubt that Ghellen can answer this question without first listening to some CDs with a good sub.

Ghellen ... try to demo a good sub. I really love the extra dimension that the sub (Rel strata in my case) adds to 80% of my CDs. It may or may not be to your taste, but the REL subs will almost certainly integrate seamlessly with your main speakers.
Well said by the way, I am using the REL Storm III.
Thanks for everyone's input. I keep learning from all of you.
Titan II LE is one terrific sub for music. Comparable to the REL Stadium III but at far less cost (direct). Their new Force should also prove to be a winner. Do some searches here, at AA and AR and over at the SMR site. Also, see the reviews in Positive Feedback, BFS.
The quality of the subwoofer must match your other speakers.Subs are good with a small speakers. The sub will make a small speaker sound more like a fullrange loudspeaker. Will the match be seemless? No it won't, but with careful set up you can get it close. The biggest mistake people make is having the sub to loud. You should only hear the bass intergrated into the music. You should have no sense of the sub being there. If you are using a sub for movies you would have it set louder. Movie dynamics are extreme.