Can you warm up your tube amp ...

Don't see why not. Warm up the amp, then turn it off long enough to hook up the cabling before switching it back on.

I thought tube amps need a load on the speaker terminals, w/o it can cause harm. I am NOT a expert here....

You should call the company for sure as sometimes it can be harmfull to run without a load.

I'm no expert, but I'm pretty sure Philojet and Jfrech are right. Last night one of my speaker wires was disconnected from my Jadis amp (cleaning woman) and the amp started buzzing loudly--fortunately caught it right away to avoid any damage. You'll need to run the amp into a load.

Some tube amps can be on without a speaker load (as long as it's low power) and some will short out at dc right away. With any tube amp (especially the 100 watt ARC's) it's best not to take any chances. Use an old pair of speakers or a dummy power resistor. But check the owner's manual for the last word.

Personally, i would not leave a tube amp on without having some type of terminating load on it. If you want to do something like this, go to Radio Shack and pick up a pair of 8 ohm or 16 ohm resistors. You can connect these across the speaker terminals like they were speakers. Check them after the amp has been on for a short period of time to see if they are hot. If they are hot, shut down the amp. The amp may be oscillating from not having anything on the input.

If you use common sense, you'll be fine and it won't cost you more than a very few dollars to do what you want to do. Sean
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For the general rule of thumb and for the safety,
First load the tube amp or any amp(plug in interconnects and hook up speakers or dummy loads) than worm it up loaded.

If you turn it off after you worm it up the rail caps will likely drain the charge faster than they would accumulate. In my tube amps it takes nearly 20 min to charge completely. The plate voltage riches 510V almost instantly but to rich normal 530V takes another 20 min AFTER you turn it off and turn back on.

If you leave home turn your tube amp off or if you want to leave it on than connect it to dummy load since if there bad tube suddenly appeared it can do very bad things to the rest of amp and speakers if you don't act ontime.

Compared to SS poweramps that may require substantially longer worm-up time you can enjoy the music of tube amp after 25min and after another 20min you will experience some improovement due to the signal path worm-up as well.

Turning on/of tube amp freequently can shorten tube life but if you leave it for long time without listening you will short tubes' listening life.

The best way to A/B speakers is to have preamp with balance control and mono mode by connecting different speakers to different poweramp channels and muting either one speaker or the other. Otherwise you should turn off, hook-up and than turn on again and play only after 25min.

You can do permanent damage to a tube amp's output transformers by running the amp without a speaker load connected - and besides, your ARC should be able to come up to it's best sound in fairly short order anyway (usually around 15-30 minutes for most tube amps, assuming it's already been initially broken-in and hasn't been sitting unused for a very long time). If you want to do A/B comparisions shorter than that kind of interval - say, five minutes for a track with one amp, then switch to the other and listen over again - just turn off the amp you've finished listening to, disconnect it, connect the other one, and turn it on (giving it 30-60 seconds before listening). This will work because the amp not currently in use will effectively stay mostly 'warm' for several minutes after switching it off and disconnecting it, so if you start with both amps from 'cold' and keep changing amps this way over the course of your comparitive auditioning session, you'll maintain a pretty level playing field throughout, and should reach near-peak (but never absolute peak) warm-up for both amps within the first 30-60 minutes. But for more extended listening with any one amp before switching (or if you just don't like having to jump up and move cables around every few minutes), you'd have to find a way to keep the unused amp safely 'on the boil' while you listen if you don't want to skew the results or face lengthier warm-up delays when swapping out.