Doh....the reason 3.6V to 4V works better with the LEDs (which was puzzling me, since the LEDS are meant to have a rated forward drop of 3.2V)), is because I have an NPN transistor switching the common ground to all the LEDs...this drops 0.7V across itself, therefore the LED only sees the 'supply voltage' less 0.7V the drop acrossr the NPN tranny) ...therefore at a supply of 3.3V the LED was only seeing 2.6V ...no wonder they were dim - man, I'm having a bad day!