I'm literally in the same boat as you :D Going to drop my bass to C before recording soon - have a regular set in that tuning at the minute, sounds far too loose.
I'd stay away from lighter gauges for drop tuning (especially since you do the same thing I do for guitar) the tuning stability tends to go a bit whack.
Low strings need that extra tension for drop tuning, I find the 105 for the E that comes on most standard basses even a little light for standard tuning.
You may need to rest the truss rod a little and most likely the bridge saddles to correct the intonation.
Never heard of Circle K but just looked them - they seem to have made the gauges for equal tension in various tunings and scale lengths
Thinking about a 5 string that typically comes with a 135 on the low B so I'd reckon either a 130 or 125 would be around the right mark for that extra half step
so either of these sets would probably do it, depending on just how tight you like the bass across all the strings:
http://circlekstrings.com/store/product2747.htmlhttp://circlekstrings.com/store/product2699.htmlCould always buy single strings from other brands if they don't have the gauge you like.
As for the brightness (although string gauge is a factor) much of that is determined by the characteristics of the instruments build,
the pickups and can be manipulated what you're running it through (ie - DI, amp, distortion, eq etc...)
Most nickel roundwounds should do the job, pure steels would brighten it up but it might be going too far for some.