Following up on this situation.
We have found the culprit!
It turns out that the original cabinet I purchased (Marshall 1960 greenbacks 25wt each) was faulty somewhere in the ohms/circuitry. This was anoying live cause I had to put the Marshall TSL60 to about 8 just to compete with the other guitarist little 40wt single speaker combo:P
The mbox didnt not like the combo and I discovered that when I took home a rental Marshall 1960A vintage 30s cabinet for some live gigs. The line-out is awesome on PTLE! I really like tones Im getting and its clear as day now:) I still would like to add Guitar Rig, but at the moment Im booked solid every week-end (live gigs) till end of summer:)
Thanks to all who contributed on this matter. luving my mbox again
PTLE 6.7cs8
Dell Precision Workstation 670
Xeon 3GHz/2GG RAM/Nvidia FX3400
PRS SwampAsh
Marshall TSL60/<cabinet pending>