The new Paul Buff Einsteins seem to be the only monoblock flashes that offer constant colour temp. All of the monoblocks I have used have this problem (including Elinchrom, Bowens and Bron).
The Einsteins are said to use a different technology for controlling the output than the others. I havn't tried one, just noticed the general buzz about them.http://www.paulcbuff.com/pcb2009/einstein.html
FLASH DURATION AND COLOR TEMPERATURE:
Proprietary IGBT allows flash duration to dramatically decrease as power settings are lowered. This is opposite to conventional studio flash designs where the duration lengthens and the color temperature falls when power is reduced.
Two distinct operation modes are available from the rear panel:
CONSTANT COLOR mode - the emitted color temperature is held constant at 5600K plus or minus 50K at any power setting
or input voltage. At Full Power, the t.5 flash duration is 1/1600 second and the t.1 time is 1/540 second. As power is reduced to ½ power the color remains constant, while the flash duration decreases to approximately 1/1700 second t.1 (note that with IGBT control, the t.5 spec is no longer meaningful, so only the t.1 flash duration appears on the rear LCD display). As power is further decreased, the color temperature remains constant and the t.1 flash duration falls ultimately to 1/9,000 second at the lowest power settings.
ACTION mode - the color temperature rises as power is reduced but the t.1 flash duration is minimized even further for maximum action stopping capability where absolute color consistency is secondary to motion freezing. At ½ power in ACTION MODE the t.1 flash duration is approximately 1/2000 second and the color temperature is approximately 5750K. Absolute values of flash duration and color temperature are indicated on the rear LCD display and are yet to be fully specified.