Can a tube amp be damaged by a speaker switch?

I am a bit confused. All of the comments are good, and are good suggestions for what you are choosing for a solution, but I think the approach is all wrong.

What Marantz receiver are you using? Does it not have Pre outs?

What Leben amp? Is it an integrated or pure amp with a pre in front of it?

I would look for ways to have the Marantz feed into the pre that drives the Leben amp or the Leben integrated and the Altec Valencia speakers as the Front L and R for theater, and solve the problem that way. Much better and safer unless the issue is that you spend much more time in theater mode, and do not want to put that run time on the Leben tubes. If you like the Altecs for speech and for your 2 channel, that would seem to be the best choice. Balancing levels when in theater mode may take a little work, but should be doable. Add an additional unity gain pre if you need more inputs for the Leben  - Altec Valencia chain.

as budjoe mentioned, and why he asked for your system specifics:

Front Pre-outs is a definite solution, some receivers have pre-outs, but not Front Pre-Outs, some do, it’s an important feature.

IF Front Pre-Outs: you would be using your Marantz to power center and surround speakers only, and the front pre-out would go to your Leben Tube amp, which would ALWAYS power the front speakers, music or video.

The Marantz would control the volume of all, including the volume of the front pre-out, and any settings you change in the Marantz would be passed to the front pre-out.

NEXT: We need to know the model # of your Leben, and if not an Integrated, any pre-amp in-line, and it would be good to know what other 2 channel sources you use the Leben for.

Modern preamps have HT bypass, simply a selectable preamp input (for Video) that bypasses the pre-amp’s volume control (so Marantz can control the volume along with the other speakers the Marantz is powering.

Older equipment, no HT Bypass: knowing it’s specifics, a solution can be found.

I use a tube amp and a solid state amp and share one pair of speakers with them. I am using a Solupeak P3 switcher. I've been using it for a while now with no issues. I just make sure I am extra careful to make sure the switcher is set for tube amp before powering on my tube amp and making sure the other amp is off. One time I powered on my tube amp without switching the switcher to it (so no load) but then powered off the tube amp right after since I forgot to switch the switcher. It didn't cause any issue, but maybe I got lucky. Realize this is a risky setup with my expensive tube amp. But the convenience is great. I also always have the volume knob all the way down when turning on tube amp and never have any source playing.

Why don't you just get a proper centre channel speaker?

In a 5.1 HT system the fronts don't carry the dialogue anyway.

@imhififan I actually have a Leben and during the course of modding it, one night I accidentally left the dummy load off and ran pink noise. Next morning..I freaked out...yet, no problem with it at all.

​​​​​​​I know there are vintage guitar amps that have blown up from not having a load, and I dont know the technical reason, but it doesn't always cause problems.