Will this isolation transformer limit power draw?

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.

If the amp is 19 years old, it still has at least a few months left of Bryston's 20 year factory warranty. In plain English, your amp has a problem. Call up Bryston, give them your serial number and see if it is still covered. If it is, make arrangements with them to send it in for a check-up ASAP before the warranty expires. Once there, Dennis at their US repair facility will check the unit out. If it needs help, he'll do what is necessary to bring the amp back up to spec. All of the parts and labor will be covered but you'll have to cover the shipping. This is a phenomenal deal to say the least. Even if you don't like / keep the amp long-term, you'll at least have verifiable proof that the amp was just inspected by the manufacturer and meets factory spec. In this regards, Bryston is unrivalled in terms of customer support.

As a side note, all the older original Bryston series of amps used "iron core" transformers. They switched over to toroidal designs somewhere along the line in their newer series of amps. Sean
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I'd be happy to solder in some new filter caps. I'll pop open the case and take a wild guess on where to find them... :) I'm getting bolder with my soldering iron in any case...

Thanks again!

@sean , at normal listening levels he would probably never hit 720Watts. So why would you suggest a minimum of 1000 watts with a very heavy core?

There's such a thing as dynamic compression & non-linear distortion.  Not only could this choke the power supply in the amp on a dynamic basis, it could introduce further distortions into the AC chain from the Iso momentarily saturating.