Sounds easy, right? Well, if I insert a Sonitus or LP 64 Eq on a mono (bass guitar) track, it processes as stereo. Why wouldn't there be mono versions of the plugs? In UAD, there's have a whole folder of mono plugs, but at least the Fatso when loaded comes up 2 channels. There may be more - I haven't investigated. The signal is mono the whole way until it hits these plugs. Isn't it right to keep a mono track mono?  Confused...

What actually happens? When you install LP 64 EQ, does your mono track suddenly flip to stereo?
Are both sides of the stereo populated with your recorded mono track?

So I tried going  changing the bus interleave to mono, thinking I was just dumb... The only thing on the bus not mono is the Sonitus Eq I put there after taking it off the track to get it to be mono. Sonar crashes every time I try to change it. Wth.

When I process audio effects, the track changes to stereo. The wav symbol changes to stereo and there is a left and right wav in the clip window.

So just to make sure I understand this correctly:
1) you have a mono track, input coming from, say, input Left.
2) You insert LP-64 EQ and
3) the input changes itself from Left to Stereo?
 
I just tried this - mine stays set to Left.
 
You mentioned busses. AFAIK, busses are always stereo, if that's what you mean.
 
Edit: Sorry, out postings are crossing each other.
> The wav symbol changes to stereo
Which wav symbol?

Not the input. I recorded the track already. It is mono. I used Ampeg SVX and did process audio effects. Still mono clip. Put Sonitus or LP 64 Eq in the bin, process audio - instant stereo track.

And with the mono interleave selected in the bus (because the bass is not tight in the middle in stereo, even though the pan is at C) SPLAT crashes. The wav symbol in track view. Right next to the track number.

Can't reproduce here. I right-clicked in the FX bin and Insert Audio FX/Sonitus, then tried LP-64 EQ, and also dragged them from the browser to the bin.
 
What am I looking out for?
Is it the same effect you'd get if you select the recorded clip, right-click/Convert to Stereo? And the input stays mono?

No, not even with interleave on stereo.
OK, I'm not helping, sorry.

You have to have mono audio on the track. You have to insert either plug and run 'process audio effects' on the track. it will turn the track to a stereo track.