I'm using 2 RCR-123A so they're only 7.4v, just wonder how it can get the best of 9v bulb? Do you have a link for SR-9? Where to get the lowest price?
It is only nominally a 9v lamp, just like 3x cr123a cells nominally put out 9v, and 2 x RCR123s put out 'only' 7.4v. It is a bit more complicated than that.
When you apply a load to a cell, the voltage will actually drop, or sag.
Under load, those 3x cr123s are putting out less than 9v, depending on the load, quite a bit less.
Lamp manufacturers know this and design their lamps to run a the actual voltage of the cells under load. 7.2v for the EO-9, and 7.6v for the HO-9.
Now your RCR123s actually come off the charger at 4.2v, x2 = 8.4v. These too will sag under load, and provide just about the right voltage to light the HO-9 just right. Note that 2x RCR123s are really to small to power the EO-9, it's current draw is too high for these smaller cells.
As a rough guide, cells should not be drained at a rate greater than twice their capacity. Your RCR123s are likely to be 750mAh, so you shouldn't run a lamp drawing more than about 1.5 amps.