Well since I wrote the post you are replying to from January 2011 I did quite a lot of digging.

You cannot efficiently send a suitable signal to an on-stage guitar amp and PA and expect to get the same sound from both at the same time.

What I do now know is a satisfactory and cheap solution is to use an active DI box which has its own independent cab emulation

I bought a couple of Behringer Ultra GI GI100 DI boxes which offer this feature. They are about £25 GBP each, but that's a lot cheaper than the nearest competition and to be honest the speaker emulated sound does a pretty good job.

So taking that approach, set up your patch with the amp block set to No Cab, stick the output of the HD500 into the DI box input, turn the DI box speaker emulation on which will take away any harshness in order to feed your PA and take a direct feed from the DI box pre speaker emulation as your no cab feed. If however you are using a traditional valve guitar amp power amp section with PRE modelling (ideally) and a standard guitar cab you can in most instances turn the HD500 cab emulation off anyway and treat the output as dry for all.  I tend to use PRE models with any real amp, but the FULL amp models should give a reasonable result when doing the same thing without the need for any DI boxes.  In other words, just let the power amp and cab handle the on-stage sound, and let your computer IRs handle cab modelling for DAW use

There's a few ways you might get around the issue and they're all valid in that if they give you the desired result - job done! It's largely a matter of personal preference and opinion. I don't think there is necessarily one 'right' way

Nick