[1000mp] Sub Receiver IF Filter

[W3OA]

I'm trying to decide if I should buy an IF filter for the Mark V sub 
receiver.

I am interested mainly in operating RTTY and wonder if  it's possible to 
select the sub receiver narrow filter when in RTTY and/or Packet modes?

The operating manual seems to say this filter can only be selected in CW 
mode but that seems unusually restrictive to me.

Thanks for any help - Dick, W3OA