stacking speakers?

Hi, I have two seperate systems, one for audio, and one for HT. I use tower speakers for the audio system and bookshelf for the HT. Due to space limitations and speaker placement recommendations ( i.e. fronts HT speakers should the same level as the center speaker) I would like to stack the bookshelfs on top of the towers. Can any one tell me the pro's and/or con's of this? I've already tried looking up past threads on this, but couldn't find anything. Thanks.
Have you not tried it yet? You're probably in the best position in determining the implications.

The only real possible problem i can see would be a compromised tonal balance due to the height of the placement and placement relative to the walls, but you may get lucky. With HT, I doubt coupling differences between stands and your speakers will be a sonic issue, but watch out for scratches :)
Placing speakers that are being used near other speakers that aren't being used will cause "sympathetic resonances". In other words, the driver producing sound will excite the other drivers not being used. In effect, they would become indirectly coupled passive radiators. This could play games with quite a few various aspects of what you hear and none of them are good. That is why a GOOD "audio salon" only sets up the actual speakers that you are demo'ing and removes all others from the room.

Using that criteria, how many "good" audio salon's do you know of ??? Sean
I overlooked Sean's good point. I assumed the monitors would be removed when not in use. Resonance is more likely to be a greater problem for music than HT in your situation.
but you can turn on the amps driving the speakers not being used, and the inaudible hiss that is barely exiting the non-playing speakers will cause it to not respond nearly as much as it would otherwise, almost eliminating the problem. it's not an ideal solution, but it can help.
I also agree with Sean's point of reradiating, not a good idea to have a "live" pair and a "dead" pair stacked. If you can't move them, the next best thing would be to get a sound-deadening cover or wrap, for the pair not in use. This may not be perfect, but is alot better than the free interaction you get now, and it may be more convenient for you to not move the speakers around. Removing them from the room is a better solution though. While on this subject, when working in a high-end audio shop, we did a fun experiment. We allowed customers to audition a speaker, and then without telling him, we then brought a cordless telephone in and set it down on the table, facing up. We acted like we had just finished a phone call and were just putting the phone down. We then played the music again, and invariably they would comment that the system didn't sound right. We used this demonstration in a "blind" way, to show the customer the effects that even a small speaker in the room can have on the sound. TV's, small radios will all have some effect. Eliminate or cover all dead speakers in your listening area and you will have better sound.
Uh, call me crazy, (and I assume you have a good reason) but if your HT and 2-channel systems are literally stacked on top of each other, why not use the same speakers for both? Am I missing something?
Thanks everyone for the great input and advice.
I'll try out the differnt options and see what happens. O.k. Mezmo, I'll call you crazy. Just kidding. I did at one time have all the speakers hooked up to my HT system, Then I found the joy once again of 2-channel audio. In novice terms ( That's me, the novice) the music sounded better in 2-channel, then it did in surround sound. Having four front's and a center facing you, didn't sound any better then when I seperated the speaker's.
I would have placed the bookshelf's to the sides for a more complete surroud effect, but for one problem. My wife, and I qoute" your not going to be spreading your speakers all over the house making it look like a stereo store" end of qoute. :) Thanks again everyone.
I have stacked speakers and found the best result to be with Aurios bearings under the bottom pair and isopucks between the two speaker boxes. That seem to isolate the resonances best. Good luck.
Hey, you wouldn't be the first to call me crazy, and the more I protest, the more it seems like an admission that I am completely off my rocker....

Seriously, though, the reason I ask is not to be a smartass, but because it is not that difficult to set up separate HT and two channel systems so that they both use the same main speakers, and I was just wondering whether you had considered it. I wholeheartedly agree that a dedicated two channel system sounds better than anything I have heard in the HT realm when playing regular old music, and I wouldn't encourage anyone to listen to music on a HT system if they are inclined not to (not that there is anything wrong with that, as they say...).

In case I’m not making any sense, (an forgive me if I am being overly simplistic), allow me to complicate matters further. Say you’ve got a two-channel system consisting of a CD player, a preamp, an amp and a pair of speakers and you are looking to expand it to include a HT setup as well. To do so, you get a HT receiver and three extra speakers (center and surrounds). The two-channel setup remains the same and, when listening to music, that’s all you need. Now, you can add on the HT gear as follows – run you HT source (presumably a DVD) into the receiver which, in turn, powers the center and surrounds, as usual. However, instead of running the main speakers from the speaker outs on the receiver, run those channels from the main L and R preout connections into the two channel preamp (into either a HT bypass loop, if available, or any spare input otherwise).

If you have a HT bypass loop on the preamp, using this setup is simple. Simply turn it on and the preamp will pass the main channel info from the receiver on to the amp and the main speakers and allow you to control the volume from the receiver. If, instead, you are running the mains into, say, the AUX input on the preamp, you’ll have to set the volume on the preamp to a set level (such as 12 o’clock), balance all of the channels using the receiver (which you’ll only need to do once), and then you can control the volume for all of the channels from the receiver as well.

If I understand correctly, you already have all of the gear you need for this (plus an extra pair of speakers). (Assuming, of course, that whatever you are using as the processor/amp component of the HT setup has preouts for the mains and the ability to balance the volume level of the main speakers independently from the other channels). For my money, this provides the best of both worlds -- you have the dedicated two channel system plus the whistles and bell of HT when you want it.

Phew, long answer to a simple question (that wasn’t even the one you asked, either). In short, if the logistics of having two pairs of speakers in the front are troublesome, don’t bother -- you only need one. You should be able to set it up so the towers are pulling double duty without compromising the two channel setup at all. Again, maybe I’m missing something, but this may be an option worth considering.
DBX: If your HT fronts and floor standers have removable grill covers, you could try placing a "solid" sheet of something (plastic or thick paper/cardboard) under them (to block the sound going to the unused speakers). This will help with the problem of the passive speakers "sucking" up the sound of the speakers being used and would be an invisible remedy, plus it would be easy to switch back and forth. Chances are that you will end up playing the speakers which are still blocked down the line (I know that I would forget and do this:-). This solid sheet should be somewhat reflective (in sound quality) and the grill covers will help to disburse the relected sound a bit. Should not be much worse than a typical "beefy" speaker stand, sound wise, and you can experiment with placement of the small speakers (on top of the larger ones).
I am kind of new to this high end stuff and usually read all the posts just to learn from those with more knowledge and experiance then myself, so this suggestion might sound off the wall but ,WHY DON'T YOU TRY STACKING THEM AND SEE HOW IT SOUNDS INSTEAD OF DEALING WITH THE POSSIBLE PROBLEMS THAT DON'T EXIST YET?
Tsugury, sure I coud have stacked the speakers first, had problems, then started asking questions, but why? Isn't that the purpose of these forums, To find answers to our questions before hand. Up until two days ago the words "Sympathetic Resonances" wasn't even in my vocabulary. I now know what it refer's to, and have several option for fixing the problem if I still decide to go that route. Being a novice on a budget, I can't afford the trial and error route.
I've read to many threads of people doing that and blowing amps, speakers, or whole systems. If it's something that I know with my limited knowledge, that won't hurt anything, then I'll do it, but why not gatherig as much information possible about something before doing it for the best possible results. What would be the use of these forums if we don't ask questions?
Sorry DBX, I guess I came through a little rough and should have explained myself better.As far as I know , there is no damage that can be done by stacking speakers on top of each other from two seperate systems ( except maybe scratchin the speakers, and be careful not to short any wires), maybe someone could correct me ?. So then, the only way I know of to see if, or how much the sound is effected is to try it out. Bottom line is your gonna have to try it, it will cost you nothing more then the time it takes to set it up. Then IF there is a problem with the sound you can identify the problem and (with the help of those in the forum) ,work on fixing it.