Will this isolation transformer limit power draw?

V.A. is volts times amps, 115V times 4.35A = 500VA. Just think of it as watts even though that is not exactly correct.

I looked at the 2B manual on the Bryston website and the AC input is labeled 720VA so I think you need a bigger transformer.

The 720VA rating makes sense since the amp can deliver 180 watts/ch into 4 ohms. If we assume that the amp is 50% effecient then it will need to draw 360 watts to produce 180, or 720 for both channels.

The hiss is normal with a lot of amps. If you can't hear it from the listening position I wouldn't worry about it. The hum from the speakers doesn't suprise me especially with an amp that old. The filter caps should be replaced in a 19 year old amp.

If you can hear the transformer humming from 10 feet away then you have a problem. If it is a toroid then it may be due to crap on the AC line. Light dimmers are notorious for causing them to hum. Even though on opposite ends of my house, the light dimmer in my stove hood would cause a toriodal isolation transformer I once had to hum. If it is a convential transformer then the core laminations may have separated. In that case you are looking at a replacement. That won't be cheap.