My tube amp sounds darker now.

I have never heard Siemen CCA tubes descibed as dark sounding, therefore, I would borrow some interconnects to try out before purchasing. Which tube preamp are you using?

Nemesis218,
What is your system (component names, interconnects etc.)?

Siemens CCa's are not considered dark sounding and generally are considered clear and neutral sounding.

The easiest check you could do is to put the original interconnects back on the amp and see how that sounds.

Leaving your amp on overnight shouldn't cause any damage to the amp or tubes. Some people believe that you should leave tubes on all the time unless you are going to be away from home for a while. That is probably a bit extreme but you should not be turning your amp off and on throughout the day. Turn it on when you start to listen each day and turn it off when you are through listening for the day.

Sounds like the cable you changed to, to compensate for the sound of the tube, is contributing to that dark sound. I found the Siemens to be anything but dark.

Let me add that you should keep your tube amp well ventilated. Don't put it into a closed cabinet or something similar. It produces a lot of heat and needs room for the heat to dissipate.