Audi exhaust vacuum pipe

[dualmono21]

i have a 2003 s4 avant which has an aftermarket performance exhaust (milltek)
on the stock exhaust there are two actuators one on each side with two vacuum pipes

the aftermarket exhaust does not have these on
does anyone know what these actuators or vacuum pipes actually do i am unsure whther or not to leave them open to air or whether or not to seal them up or worse case scenario actaully have the valves welded
onto the aftermarket exhaust

the car seems to struggle a little just before 2000 rpm in top gear if i leave the vaccuum pipe open
but if i close them off i get loads of error messages in vag com for both brakes and engine

any help appreciated

 

[quattrojames]

Where abouts are these actuators?

The best I can offer is THIS

 

[TorqueTerrorist]

These are called active exhausts are they not? They open up fully when at full throttle to make it a more straight through system for performance and sound. Being a high performance model not everyone would want it to be loud all the time i guess