How many gain stages is not the same as how much gain ..... you could have a amp with 3 gain stages that has a lot more gain than a amp with 8 gain stages.
You really need to play on these amps side by side with your cabinet & your guitars to really find what one works best for you. I had a CLX, CL, UL, D60, D120 & Sig-X all in the same room with my gear for several days & I much prefered the D120 & Sig-X ... but like all of the Fryette amps - only a matter of taste.
If you ask me the Pittbull's are bit more modern voiced - the Deliverance & Sig-X also can be e very modern sounding when set up that way- but have a bit more of a vintage flavor & feel is bit different.
If you really want to know about the differences ask more direct questions - Writing out all the differences of this many amps would take weeks ...