I have Klipsch F4s and the Klipsch Sub 12...I have limited space in the corner next to my entertainment center.  Is there any absolute no-nos as fas as placing the F4 on top of the Sub...

 Still learning...

Art S.

F4? Do you mean the Synergy F3 floorstanding mains? or perhaps the Icon XF-48?

Either way, you're not going to damage your subwoofer, but your subwoofer cabinet isn't going to be without vibration. This will cause your main speaker to vibrate with the sub, which is going to modulate the sound coming out of that speaker (basically you're vibrating the cone that vibrates to make sound). This will cause distortion in the midrange, causing things to sound a bit blurred when the sub is cranking away.

Also, how are you going to get the left and right side to be at the same height? Being unbalanced will cause all sorts of imaging problems.

I might suggest finding yourself a little table/stand that can fit over the subwoofer that the main speaker can sit on - and then get two of those stands so that things are the same height on each side.

F4? 

 Art,

Welcome.

Even though it would be out of place, I would set if off to the side of the speaker, if possible..........JMHO

Does it look like this picture?

 I would not. Safety issue, plus some blurring of the sound caused by the cabinet vibrating might occur. THe tower should be tightly coupled to the floor with the spikes.

Yup -  I meant the F3 floorstanding mains.  I have opted for the table/stand(s) you suggested and they are working fine.  Found some heavy ones made of metal - they are actually patio tables made for the outside, but they work great and I like the solid base they provide for the speakers.The tables also bring both left and right speakers up level with the arms of the couch and love seat on either side.  I can actually hear a significant difference in the sound.  Thanks to all you guys for your suggestions...

Respectfully,

Art S.

So you have your mains on stands?  If so, I would try to take the stands out.  You want the horn as close to ear level as possible.  I'm pretty sure my F2's would sound pretty nasty set up like that..............

Actually only raised them about 19 inches...Sounds great to me!

Well, if it sounds good to you, then that's all that counts!