I would recommend against overlining the RPLXi, especially with full head lines like the clear intermediate 444 from Cortland (a good northern flats line), the mastery steelhead long belly floater (a good singlehand greaseliner choice) or the other good lines that people recommend here. The thing with the RPLXi is that it is a very crisp rod, but not stiff. You need to find the sweet spot and it will do all the work.
When I worked at a fly shop I pushed a lot of RPLXi 9wt rods because I truly believe in the rod, and fish with it. It is not a stiff rod, but it is fast. Many do not separate the two (fast, stiff) - and there is of course a relevance, but a fast rod does not need to be stiff. I hate stiff rods, others love them - so there is the preference thing as well. I like a rod that really loads up but doesn't buckle under the load. Stiff rods don't load well, and take a tool on the appendages - but they can throw (stiffness provides modulus). Soft rods overload and also can't launch the energy (no modulus). To me, a well engineered taper and the internal structure of the blank lets you really feel the cadence of the cast at the loadpoints but when you stop the stroke the energy just rips out of the rod and into the loop. The unloading energy is fast, but the rod is not stiff. That's why I love the RPLXi.
I've been the benefactor of the Sage Pro program and I feel I have the obligation to put my money where my mouth is. I won't call it a lesson but you can count on me working with you to find that natural and easy recoil point that the rod has. Once you do, it's just a matter of loading up the catapault and letting it rip until it hits the block that sends the rock straight, far and true.
You tell me when and I will make time. I'll bring my Rplxi 9'er as well so we can experiment with various lines easier without changing the rod.
There are other rods that transmit the loadpoints more emphatically, for instance mid-flex rods. They are nice for small trout streams but won't cut it in a hard wind, big flies and big fish that 9 wt rods are associated with. Stiff rods do a terrible job of communicating their load dynamics, at least the way I feel my way thru a cast.
I am certainly no expert but I think I can help you find that magic in the rod.
Maybe we will pull off this indoor casting clave afterall! If anyone finds a roller skating rink, a large hall, or a gymnasium in a central location, I think that would deserve a Forum service award (hint hint). Maybe a guided trip from our sponsors? Maybe a killer flats chest pack with hydration system from Prof. Bodkin?
Anyway, didn't mean to digress. Let me know when and I will be happy to see if I can help. I'm sure others will too. I noticed BobP was all smiles with his Rplxi last summer...