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?
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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.
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.
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.
Just wanted to add my $0.02 to the conversation. I agree with others who've said something doesn't sound right when you say the Siemens CCAs sound "dark". I just installed four Siemens 1960s vintage, grey plate, silver pin, 6DJ8s in my Manley Chinook. All I can say is WOW. Their reputation is well-deserved. Siemens tubes (genuine) don't sound anything like "dark". If you can return them, that might be an option. Contact Andy at Vintage Tube Services - he stocks Siemens tubes.