I appreciate this is an old thread but I find myself in a similar situation. I already own a Rumble 500 - it has a compression horn driver by the way so it can handle high frequencies. I also own a Helix Floor and an HX stomp, and am looking at swapping that out for a NeuralDSP Quad Cortex. I also own a Headrush FRFR (as I also play guiter through the helix) which is woefully inadequate for Bass. I did just find this though which looks interesting - will give it a go later:
https://www.talkbass.com/wiki/fender-rumble-500-frequencies/
