My tube amp sounds darker now.

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.

As Columbo would say, Uhhh...just one more thing.

The good CCa's are from the early to mid 1960's. They are identified by a grey shield or riser. Later vintage CCa's (late 60's on) have a shiny shield or riser and sound brighter, even harsh. If you look around the plates in your CCa's and they are grey all around, those are the good ones. If they have a shiny (silver) strip on opposite sides of the plates they are said to sound much brighter.

It may take some time and experimentation to get your amp and system dialed in to your liking. If you turn your amp off before changing tubes and follow normal safety precautions you shouldn't harm it or yourself. So take your time and make changes to cables and tubes until you get it right.

Also follow the manufacturer's instructions for biasing the power tubes. That shouldn't be too hard either.

If your Siemen CCAs are dark, then they are PROBABLY FAKE!

From my experience, they are the most open, dynamic ... compare to my stash of Amperex, Tele, Siemen ... 7308/6922/e88cc.

My tube amp sounds best after it has been on for several hours. Aural memory is very unreliable- listen further and see if your ears adjust before making changes. I've listened to systems I thought incredible, and on later re-asessment I found them bright or forward or rolled off. I only change things when a I notice a pattern or tendency that spans several sessions.