Can I connect two amps to the same speaker?

Do so at your own risk...It is NOT advisable to connect 2 amps to 1 speaker...

Why not.

For obvious reasons...

You stand the risk of running current driven by one amp into the output side of the other amp, something that isn't intended by any stretch of the imagination...

DOes your receiver have pre-outs? If so, you could use the preouts for the front LR from your receiver and plug them into your power amp and then use the power amp to drive your fronts for both HT and music. You would of course have to route your 2ch music through the receiver in that case.

Its like crossing the streams, you just don't do it. It would be bad. Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light. Total protonic reversal.

don't try to reinvent the wheel, use google
http://www.hometech.com/audio/spswitch.html

Don't even try it. Just run pre-outs for the front left/right to the amp and use it for both surround sound and 2 ch. I don't understand why you would want to use the receivers amp for surround sound and the external amp for 2 ch. music.

You can do it. But you must get a dual voice coil speaker. Not even sure if those are still made. I've seen them only on woofers.

Major thread necromancy here, but this question recently came up and I wasn't sure what to say.

Surround sound AVR and a very nice tube setup (separate pre and power amps). He wants to use the same front L/R speakers with both amps without switching the speaker wire each time.

Would a simple a/b switch work? This way it would be impossible for them both to be connected at the same time, and therefore avoid any backflow on the amplifiers.

Yes.

An A/B switch will work but get the "break-before-make" variety if you can to keep from momentarily shorting the amps (most speaker switches are already designed that way). Also, not all amps are stable into an open load, and tube output transformers in particular may get warm. A double-pole, double-throw relay or switch would be best, with a load (can be fairly light, like 16 or 32 ohms) to keep the "off" amp happy. That said, tube amp outputs are generally more forgiving of brief shorts and the like due to their transformer isolation.

Edit: Kal gave the short version of my answer.

As for why not connect the wires together, the amps are not always in phase, and many amps have floating outputs that really don't want to see anything except a load across their own two terminals and nothing else. That means that, despite appearances, the amps are not always (or even at all, if the phase on one is reversed from the other) going to be working together in parallel. There are also load-sharing effects, stability issues, etc. Think about taking a power cord from the wall, putting a plug on the other end, then plugging it back in, backwards. Or, jump-starting a car with the positive and negative cables backwards on one vehicle. Don't try this at home, folks...

Niles Audio makes a the switch you need.

http://www.nilesaudio.com/product.ph...ecordID=Source Switching Systems&categoryID=Switching Systems&catcdID=10&prdcdID=FG00003