Will this isolation transformer limit power draw?

Much of this will have to do with the accuracy of ratings, both from Bryston and from Stancor. You also have to remember that momentary draws can be quite high when trying to reproduce dynamic peaks. While the 300 watt draw from a 500 watt transformer does follow the "60% rule" that i spoke of in another thread in terms of rated load and draw, i based that on having quite a large ( heavy ) transformer core. Given that i'm not real familiar with the Stancor's, i don't know if they fall into that category or not. I know that their smaller transformers seem to be reasonably well built, but....

Call up Stancor and or look on their website. Look to see what the transformer is rated for in terms of noise reduction / attenuation and internal capacitance.

As a side note, Bryston actually uses a little more filtering in their power supply than many other amplifiers. That is, according to their schematics. Then again, Bryston's were originally designed as Pro Sound amps where AC is typically "worse" than in a home, so they might have taken this into consideration. If you've got an old original 2B, it will have an iron core transformer, not a toroid. This puts you quite a bit ahead of the newer designs in terms of the noise floor of the power supply itself. Sean
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I can find this information on the datasheet:

802-GIS-500 Isolation Transformer
Input 115
Output 115
Amps 4.35
V.A Cap. 500
Wt. (Lbs.) 16.0

I suppose I will have to call Stancor to find the figures you mention - I'm not sure what V.A. Cap. is...

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 transformer is probably OK for any reasonable home use of the amp, but why the heck are you using the transformer?

You need a bigger transformer. With a max current rating of slightly over 720 watts, the absolutely smallest transformer that you should think about using should be at least 1000 watts with with a very heavy core.

You can always try using the transformer if you already have it, but if you haven't purchased it yet for this specific purpose, you should thank Herman for saving you the money. Sean
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