What would you use if you had 5 levers and 4 pedals? Which pedal and knee would you leave out?
It’s an interesting question. I’d probably do a S-10 4+5 hybrid D6th like this:
LKL LKV LKR P1 P2 P3 P4 RKL RKR E +F F# +G D +D# ++E ++E -C# B ++C# +C/C# A ++B -G# F# --E +G D +D# (-C#) B +C G ++A +G# D ++E
Imagine for a minute that your E9th string 2 was tuned to C#, and your half stop lever raised it to D and D# (like Weldon Myrick). Then imagine moving that string to be string 4 instead of string 2, so that your top 6 strings are F# G# E C# B G#. Then lower it all a full step to be based in D instead of E. That’s how the hybrid D6th works. P1, P2 and all of the knee levers are standard E9th changes on the top 7 strings, but the strings they work on are tuned one step lower.
This tuning doesn’t have the 7th and 10th strings of the E9th. It has the middle and low end intervals of the C6th instead. It’s a hybrid, not a universal.
With just 4 pedals, you’d be missing the C6th P6, but you can get that essential change with P2+LKR and a tuneable split on the 6th string (which is what E9th players usually do anyway). It feels better on a pedal, but if you only have 4 pedals you can still have that functionality.
With only 4 pedals, it’s totally missing C6th P8. The low P8 “boo-wah” changes could be added to LKL (the E9th “F Lever”) if you really need them, but they would be better on another pedal.
The high half of the C6th P4 is there on RKR (string 4). I also have the 8th string full step raise with a half stop on my RKR, but it’s pretty tricky getting strings 4 & 8 to half-stop in sync on most guitars.
On standard C6th there’s a high C to B lowering lever. The equivalent E9th lever lowers both E strings to D#. So, on the hybrid D6th I lower both D’s on RKL. I back out the nylon tuner if I’m playing a “mostly swing and jazz” gig.
I hope this explanation makes sense. In the hybrid approach, you don’t have to hold a knee lever to switch from “E9th mode” to “C6th mode”. You are always in a D tuning. You go to the leftmost pedals for E9th licks and the rightmost pedals for C6th licks. The knee levers are shared between the two approaches.