For a cheap but sounds good alternative; grab an old 90s Digitech GSP 21 or related units…they measure up well even compared to the AX8 that I had a few yrs ago.
Tons of preamp pedals put these days, but a little harder to find some high gain ones. Tubesteader makes the sun keeper, which is an orange rockerverb 2 channel tube preamp pedal, may be more suited for doom than what you want but a good option. Also a couple of Mesa studio pres are in the for sale section here.
I've tried more than 20 high gain preamps and my favorite ones are:
Mesa Boogie Quad (really raw and in your face)
Mesa Boogie Studio (more scooped / modern sounding than the Quad)
Peavey Rock Master (Really versatile )
Kettner Access (The triaxis killer)
Rocktron Piranha (Love - Hate thing )
ISP Theta ( good and cheap sounding SS preamp)
^^^^ for sure! Dialed in properly, it screams. Triaxis with eq in the loop kills too...if I was to go modern day it'd be synergy or the black widow preamp
The Axefx FM3 blew me away and I normally dont like digital preamps. Its an all in one small pedal board size. I run that to my KSR tube power amp through a regular 4x12 guitar cab. Not sure what your application is but its a great for a grab and go rig with a band.
Rack tube preamps:
Mesa Quad
Mesa Triaxis (You'll want an external EQ if you go this route. The Triaxis sounds great but the built-in EQ called "Dynamic Voice" is awful)
Pedal Preamps:
Bogner Ecstasy Blue pedal
Bogner Ecstasy Red pedal
Carl Martin Plexitone
Friedman BE-OD
JHS @+ (Andy Timmons pedal)
Joyo American Sound