I'm also a Rockabilly player. I'm running an Aguilar GS 412, Iamp800, Shen lami bass or German lami with an Underwood and guts ( plain up top & Olivs on the bottom). D-tar Solstice sometimes, dependant on the bass I'm using and the room we're playing in.
One of the bands I play in is a really loud Rockabilly band, and with this setup I have no issues with volume or feedback, even in larger venues. My bass sound is natural and shakes the floor, happy days...........
I've had loads of amp & cab setups over the years for Rockabilly, most of the time and old Ampeg SVT3pro or Ashdown ABM500 with an old trace 1048H & had good results too. Good luck, my sound quest has been going on for 20 years+, but I'm happy with my setup (for now). Only downside is the weight of everything, but in my book it's worth it.
Also as Bob says above, a good compromise can be mixing a mag pickup and a piezo, I also did that years ago, but I just love the sound I can get with guts and an Underwood. If you really need to have the loud backline like I do, there's loads of info on fighting feedback over at Rockabillybass.com, or PM me if there's anything I may be able to help with.