Hi - Congrats on the new rig!
Ours too is an 04 Allure. Like Bob said, the switch to the left side, rocker back towards the rear is "low", rocker forward is "higher" (we have the two stage Jake's, so only two positions).
Our rig only had 13K miles on it when we bought it. The 1st owner was 'get in and drive' hands off onwner. We've been slowly doing the new coach shake out on this 2004
! We found very litte difference tween low and high on the jake while test driving. As part of the agreement to purchase, Cummins went in and checked our Jake over for us. Found that they had to pull the valve cover, and reset a mechanical portion of the valve train that close the valves - in high, the valves were remaining partiall open, thus little to none difference with on the first band of 3cyl's working at low. After the repair, the 2nd 3cyl (all six now) did kick in during compression braking. Very nice for those down hill grades!
I do recall reading about some pin/plug problems too, where some Jakes were only working in low, not high. Not a factor for us, but worth a mention.
Suggest you go out and find a good size hill, slow the unit down to 50, and kick in the low Jake, see how it feels. Then kick in the high, and see if you feel the difference in the seat of your pants. If not, then look further. If so, then grin and go have some fun!
Best of luck, be safe, enjoy,
04 Allure 40' High Cascade 31017