I interpreted you as saying that it was pulling the machine down when in use. A 1666 is a smaller machine than the 88 series, therefore, if were pulling it down, I thought maybe it was to the lack of horsepower. This is becuase I have never seen a mud hog on anything but an 80 or 88 series combine.
This occurs with all RWA axles on the market, you are diverting hydrostatic oil two ways now instead of sending it all to the front ground drive. The ratio of ground speed lost depends on what rear axle you have, what motor displacement it has, etc. As said above, generally count on around a third less speed when it is engaged.
The two speed mentioned could be an option on there, but with a 1666 I doubt you have it. This of it as a dual range, splitting the hydro into high speed, lower torque and a lower speed, higher torque. It is quite common on 2x88 size machines, but dont' see it quite as often with the smaller ones, especially in the 1600 and prior machines. It is an on the go shifting deal, just flip a toggle switch one way or the other, no adjustments or anything of that sort.
Keeping the RWA engaged in the field will actually help you give more power to the guts of the machine as less is used with the ground drive. Some people claim that the machine will turn at the ends easier, myself I do not see a difference one way or the other.