two amps/one set speakers

I would like to use one set of speakers for both my 2ch (tube amp) & HT (SS amp) systems. Without using a switch box between the two, can I simply keep one set of speaker cable spades hooked up to the off amp and then insert a second set of speaker cables from the on amp (SS) via bananas. In other words, both amps would be connected to the speaker posts but one amp would be on and the other off. Is there any downside to the following - besides remembering to keep one amp off at all times?
You should not do that in most cases. Some amps have output relays in which case you "could" but just to be safe, I would advise you simply change cables when you want the other amp. This will guarantee that you won't have any problems.

have you considered playing your HT Mains through your 2 channel preamp? It solves the problem of shorting an amp, but (I guess) adds a switch. You don't have the expense of the switch, and I suspect any "contamination" added to your HT sonics by your preamp will be negligible. It requires pre-outputs from your HT rig however.

Here's a thread that discusses it in detail (there have been many others like it - search on HT):

I've been running mine this way for some time, and it's the only way to go.
A little clarification to my post. Up until recently, I have been using the HT bypass in the preamp to control both the 2ch and HT - when I had only one amp. Yet, I prefer SS for HT. So, now I'm looking for a simple 'work around'. As Aball points out, I might need to switch cables if there's a chance that I can damage something.
In many cases, you can damage something for sure. What happens is that the amplifiers' output impendances are very low - like 0.2 Ohms. If you connect two of these amps at the same point (the speaker), then you are effectively paralleling the amps. This means that if there are no output relays, when you turn one amp on, the path of least resistance will be the 0.2 Ohms of the other amp - and not the 6 Ohms (say) of your speaker.... Most amps will blow up if you connect them to a 0.2 Ohms load!

Tube amps have a higher output impedance but I still don't advise you try it. Connecting two voltage sources in parallel will always cause damage of some kind.

Its not even just damage to worry about, the sound will be backing up thru a passive circuit in the other amp causing major impeadance changes, this is thru the Dead amp sitting on the end of the line that is, cause your signal will still pass thru the circuit from the binding posts backwards thru the other amp that is not being used..

The damage can occur especially with tubes if you forget to turn one amp off before turning the other on.

Myself and a friend have experimented with this method, totally screws up the sound and adds un-needed resistance, especially when running the Tube amp, it sounded like one channel was scratchy, and the other channel had no bass, once we disconnected the HT receiver "on the fly" even though the HT was off at the power supplys, the Running tube amps completley straightened out and were perfectly balanced again immediatley..

So yes no matter what it will screw with the Tube circuit it seems when having the Dead circuit at the other end of a solid state amp in our experience connected to another set of cables sharing the binding posts on your speakers.. And even if in your system its not audible for some reason that you can tell, I would bet it would not sound as good or be as safe as it should be...

Only thing is to run a switch box of some type to disconnect that amp physically or pull the wires off the amp not in use each time.. Real pain in the A$$.
If you’re concerned about degrading your signal path by using a switch box, you may want to consider building your own "high-end" switch box with ultra quality components (i.e.: wire, circuits, connectors, etc.) Google "how to build an audio switch box" to gain access to a myriad of information (more than you'll want to know - isn't that always the case). You'll see that building one is relatively easy and you'll be able to "hot-rod" yours all you want. Maybe you can throw in a little silver wire, some $5.00 circuits and a few gold platted connectors then market them as the latest tweak for $2,000. You know someone would buy it.