If there's an audible difference between the volume model and the standard model, I'll give you a Golden Ears award. There's obviously more "stuff" in the volume model, but it amounts to an extra inch of wire and a potentiometer. If you plan to use the volume control, get that model. If not, get the standard unit.
The volume model gives you the opportunity to eliminate your preamp, but I doubt that will give the best results. Are you familiar with passive preamps? That's what the 834P volume becomes in this situation. In theory, it seems smart to reduce the number of components in the signal path, but an active preamp does something the 834P doesn't do. It controls the impedance of the signal running through the cable and keeps it stable. Passive preamps are notorious for sounding thin in the bass and mid bass because they can't control these impedance fluctuations. It's best to hear these things first-hand rather than from some stranger. If you have the opportunity to listen to a passive preamp, you should do so before deciding on the 834P.