I just took a look at your system and all I can say is WOW, nice setup! Are you asking this question because your 801s are too big for your tastes or room or just curious about an alternative configuration? Your question has been one of the ongoing debates in these forums. It's not a dumb question at all and should inspire some great responses.
There will be those like myself that use monitors with a sub because of the great soundstaging capability and ease of placement of small monitors combined with the versatility that a movable/adjustable sub bass system provides.
On the other hand there will be those that prefer floorstanders because of the potential difficulty of tuning a subwoofer to match monitors seemlessly or they may feel that full range speakers offer a more coherent sound.
I've never had floorstanders in my system so I can't offer any valuable insights into their setup. I enjoy small monitors because of their ease of placement and because I can own more than one pair for about the same price of a good floorstander.
So is a monitor/sub combo as a good as a floorstander? I think it depends on what you're comparing and how well each is setup.
It can almost do it, provided that you can cross your sub in as well as the designer of your speakers. With full cognizance of all the parameters involved. This is unlikely. But, is a possiblity. The other problem is, that subs, in fact, are directional, and the location out of the line-source will make a difference, and turn your system into an "array" instead of line-source. Typically, this affects imaging, in this case in the bass region. This is not to say that you couldn't get an acceptable result, and many people are happy with the results they get.
"if you add a subwoofer( assuming a good one) to B&W 805 or Martin logan entry level model . do you get the same performace as N801 or prodigy or more because the subwoofer in many instance will go even deeper than the full range speaker?"
"if not why not?"
The 801 has a different xover (maybe even using better parts) and it uses a different midrange driver (the surroundless ones whereas the 805's have a surround made of rubber). The Prodigy should also do more in the top-end and midrange than the smaller logans. The same tweeter in two different systems can sound different too. Depeding on the sub you choose you may get more bass extension than the bigger model of the same manufacturer, but it turns into apples v. oranges at best. There's also the phase issues.
If your question supercedes B&W and ML, then I can tell you I am using Reference 3A MM de Capos with a SVSubwoofer- total retail $3200. I have owned Genesis Vs ($12K), Accoustat 2+2 and 3+3s, Chateau References (monitors) with dual Sunfire MK II subs ($6K+). The de Capos SV Sub combo is by far the best I have had. I will add a second sub for stereo. My room is 19X 31X 14.
If you are interested in trying this combo, my de Capos are for sale (see ad/auction). I am a dealer and getting their new "i" version.
While the above points about sub integration are valid, they fail to mention one of the biggest advantages of the sub/sat approach: the ability to site the sub to optimally deal with standing wave problems, and to site the sats to image optimally. No full-range speaker can do this as well unless you happen to be incredibly lucky. I would add that the only subs I would consider are the RELs, which are designed to cross over extremely low and come in under the natural rolloff of the sats. I am not a dealer or otherwise affiliated with REL, but in my experience all other subs have problems integrating seamlessly with the sats, probably mostly because they cross over much higher and/or put the sat signal through a crossover. Until REL came along, I would have said that sub/sat systems were hopelessly flawed, but not anymore. Lastly, try to find a reasonably sized full-range speaker with flat extension to 10Hz. On balance, I would say that properly done, a sub/sat system will significantly outperform a full-range system of equal cost.
"the same performance"?
From the perspective of soundstaging and image control, you may well get BETTER performance. I would have to pay a LOT more to get what I currently have with my Nautilus 805s and my REL Stadium III Sub. I personally do not find the 805s to be all that light in the 30-50 Hz range (partly because of the amp I use) I would like to lose the metal dome tweeter, however.
If your goal is to reproduce the cross-over designs of the 3-way, I would expect the answer to be no.
If your question has to do with whether or not you would like what you HEAR with monitors or planars plus sub, the answer might be yes.
In my price range, I have to live with trade-offs, and this is one that personally pleases me.
Well said, Karls.
I just happen to sell a sub that may blend even better than the RELs. It's the Tegmentum sub from Buggtussel.
The Tegmentum is a transmission line sub with claimed response to 16 Hz. I can't independently verify that, but it does go very deep and is very quick and tight.
The Tegmentum is designed to come in under the main speakers, like the RELs do. The crossover is continuously variable in frequency from 35 Hz to 400 Hz, and has variable slope and filter Q. In addition, the amplifier/crossover includes a two-band parametric equalizer. This is the most flexible crossover/equalizer/amp I know of, and that flexibility is quite welcome in getting a good blend with a wide variety of main speakers.
I wanted to be a REL dealer until I encountered the Tegmentum.
Duke, haven't heard it but sounds like I should. My only concern is that 35Hz is often not low enough, even for an average-sized monitor speaker. The RELs will often be rolled off in the 20's even with relatively small monitor speakers. Perhaps you can persuade him to cut the frequency range in half, to go from 17 to 200 Hz. Then you'd really have something!
Karls, Buggtussel will do custom frequency ranges. They're a small company enough that they're quite responsive to individual needs.
I can't say a smaller speaker is going to image better. The only distortion a smaller speaker intrinsically cures is less diffraction. Cabinet resonances also mess up the sound (the 801's and 802's have ceramic heads with non-parallel sidewalls whereas the cheaper models are still wood "boxes.") B&W was using concrete for some of the heads in the original 801 Matrix series. Not to mention when the designer knows they have the luxury of the LF driver they can set the crossover point a little higher than a usual sub setup and take some work of the midrange driver, reducing intermodulation distortion in the midrange and opening up new design options-like the "surroundless" midrange (which obviously have very low excursions and are physically limited from going very deep at all--hence I've never seen one in a 2 way). The bigger better models are typically using better drivers in the mid to upper frequencies which helps soundstaging, imaging, and everything else. Personnally I don't think B&W are that great--but its an example to kick around a few generalizations showing what one manufacturer did. (There's always the issue of how well they did it, the specifics may be the next model up does kind of suck, but....)
Imaging also becomes a function of the speakers off-axis response and room interactions. These two imaging properties tend to exist separate of baffle size and even driver quality. 3/4" domes have better off-axis response than 1" domes in the highs, cones have better off-axis response than domes, for better or worse [Colloms just says "there's some debate on issue" and drops it there in his book. However, I did see an AES paper by another individual that specifically addressed broad off-axis responses v. beaming, using the word loosely]. Not to mention a big baffle can be covered in foam to deal with the diffraction (Dunlavy). I'm not sure on the room standing wave issues. A sub can be placed optimally for the standing waves. But, I've also heard two subs (or two full-range speakers) will tend to smooth out the room nodes as opposed to a single lf unit. I don't think there are any generalizations that can be made on the issue, it comes down to specific designs. I won't deny some sub/sat systems can do a better job than some full-rangers--usually if you're willing to jump product lines and mix-n-match. But if you're talking within a manufacturer's model line the bigger unit *should* be better across the board. I won't deny adding a dedicated sub can probably get you deeper bass though than many "full-rangers".
maybe 801 /805 wasn't such a good example. but that say if you use the same midrange and tweeter but one has a smaller woofer 6 inch vs the full ranger model 12 or 15 inch. I think it would cost much more to build a 12/15 inches full ranger speaker and it still may not go as deep as a power subwoofer that crossover with smaller speaker . many powered subwoofer can reach 20hz where as alot of full range speaker don't get anywhere near there. I think the N801 goes down to 37hz.
Obvious you don't get someting for nothing. if it were that simple , manufacture would sttop building full range speaker , and we all be using sub/sat for our system.
" I think it would cost much more to build a 12/15 inches full ranger speaker and it still may not go as deep as a power subwoofer that crossover with smaller speaker"
True, but there's more to bass than depth/extenstion. Like output (i.e. how loud) and quality (i.e. distortion). A low system Q always means a big box, especially with acoustic suspension or infinite baffle. And some subs use eq. and other gimmicks to trick thier output in small(er) boxes.
The 801's: that 37hz is more or less still flat. At 23hz its down 6db; and -3db at 29hz--not bad. Room gain should pick that up to give it a smooth in-room response pretty much down to a reference 20hz. Alot of powered subs can go lower. Its gets into specifics, like distortion levels. Its a good speaker to supplement with one true subwoofer that'll cover 40hz on down...18hz, 8hz whatever. I really have a hard time with anyone complaining about stereo bass at 25hz.