To do a photo like this, just now of talented cellist Kendra Grittani, you need a lot of flash light:

So if one speedlight into an umbrella does not give you quite enough light, you can:

– move the umbrella closer to the subject
– turn up the power
– add more flashes

I did the latter. One more flash at full power on a separate light stand, but aimed at the very same umbrella. It worked. One stop more light.