I haven't finished my coffee yet so I'm not going to try and understand your signal flow
I play bass with the FM3 from time to time. So I created a bass preset that has something that you may find interesting.
First, a quick note - the bass player in my main band often used a Fishman bass pedal that could produce an octave + fifth signal to a separate output and we'd run that into a Tech 21 Plexi pedal and later an Atomic Amplifirebox. Then he got an HX stomp and we created a single patch with a pitch block and two amp blocks to do the same thing. The reason for this is that we have usually just been a 1 guitar band and he would kick in the faux guitar during solos when the bass is pumping 8ths or something similar.
So I wanted to do the same thing with the FM3. But there's just one amp block and I use that for an SVT amp. And there's just one cab block that has a matching 8x10 IR. The solution for the Amp block in my case was to use a Tube Drive (I think I ended up with the 3-knob version). For the cab block, there's two ways I could have done this.
- Take advantage of the fact that the cab block is stereo and you can use two IRs. Pan the SVT hard left and Pan the Tube Drive hard right - then adjust the balance in the cab block (this happens after the IR have been applied.
- Use a technique that @FractalAudio shared a while back that uses a filter and a parametric EQ to do a faux cabinet. I'm guessing it's a similar approach to what something like a Palmer PDI-09 does.
The filter and EQ uses less CPU and it's already being fed a very dramatically pitch-shifted signal so it's not exactly going to sound like a real guitar anyhow.
With the faux guitar and amp mixed just enough to be heard but not dominant, the end result is actually pretty good in a live mix - obviously this isn't something you'd do if you were recording.
So the tl;dr reason I shared this, is that you may be able to do a similar thing with the FM3 to produce your sound. Run the lows through an SVT amp/cab block and then run the highs into a Tube drive with either the filter or stereo cab technique. Since you're using a 5150, I'd use the 4-knob tube drive - it has a lot of gain on tap.
How do you split your highs/lows? Crossover? Low/high pass filters? Or?