Logic Pro 7 & earlier Stereo compressor problem in Logic 5 and 6 (but not in 4)

I'm having a problem with some stereo plugins that were working fine in Logic 4, but in 5, as well as 6, they seem to cause the treated stereo channel to go bonkers after a while.

For instance a stereo compressor plugin (in a stereo channel, of course, interleaved file) will sound like it's artificially widening the stereo spread when activated, and after a while of looping there are digital-distortion sounding artifacts being introduced, gradually driving the level into the red, eventually pulsating periodically. Bypassing the plugin will not cure this. Only killing the plugin altogether will stop the digital overload, the pulsing, and the oddly separated stereo spread. Other Logic stereo plugins I've tried (Chorus, Limiter, different compressor, etc.) will show the same behavior.

Interestingly this happens only when using my Sonorus STUDI/O card, but not when using the built in Mac AV!

All of this is NOT an issue in Logic 4.

Anybody remember coming across an issue like that?

(Mac OS 9, G4 450 - and still getting a lot done!)
Hmm..., if nothing else, at least as a record of my findings about the distorted plugins, in case someone might benefit from it, somewhere down the road, here's what I found:

In Logic 4 everything works without a problem. But it seems that starting with Logic 5, and as well in 6 (up to 6.3.1 or even non-Pro 6.4.3), something has changed in how Logic's plugins address at least MY soundcard (Sonorus STUDI/O). In straightahead playback everything works fine with the STUDI/O, with the built-in Monitor outputs as well as Optical Outs. But as soon as any inline Channel plugin is introduced (into a STEREO channel/INTERLEAVED stereo file, not in Mono!), e.g. Silver Compressor, I can hear digital distortion, as if something is clipping. Subtly at first, but it keeps getting more noticeable as the looped region is playing, even though the level meters are all in a perfect safe zone. Even bypassing the plugin will not change the "clipping spikes" sound. Only physically resetting the Insert to "No plug-in" will eventually get rid of it.

This is with Logic's own plugins, as well as with some WaveShell-VST compressor I tried. Same phenomenon.

All of the above is NOT an issue when running through the built-in Mac AV Outs, so it doesn't seem that the plugins themselves are at fault.

The STUDI/O card works perfectly otherwise, so I don't think the card is fried.

This is all in OS 9.
The STUDI/O went out of production before OS X came along, so it's not useable in OS X anymore. Therefore I can't tell if this issue would have been different in a newer version of anything.

The problem seems to come down to how post-4 Logic versions make the Stereo PLUGINS handle the soundcard Outs. (I have no other card to try out and compare things.)

* * * * * * * *

Here's a workaround I found:

Rather than sending the problematic stereo channel directly to the desired Outs, if I assign it to a BUS, make it stereo, and send that Bus out to my Outputs of choice (including the affected soundcard), and if I then use THAT BUS's Insert for the stereo plugin - NOW IT WORKS!