I hate to have to ask this but I get so confused with all of this amp talk

Would you mind explaining real quick what this would do and what benefits would be gained when adding it with a head?
You mentioned it was part of Rex's sound so that is what drew my attention

I love Rex's tone and I was always curious how he achieved it but there seem to be so many different explanations. If it's too involved, don't worry about it...I was just curious.
Any "head" has a preamp section ( where the signal is made / shaped) and a power amp section ( where the signal from the preamp is amplified ) . In most heads they are in one box and can't be separated.
A separate preamp and power amp are just that, basically two separate boxes that make up a "head"
The ampeg stuff, the svt2 and 2 pro heads have a pretty unique tone to them. Their also all tube in the head version and weight 80 lbs and are only rated to 300w I believe. To me, the main part of that tone come from the preamp, there are no transformers or anything in there so it light ( less than 4 lbs). I wanted that tone but not the weight and also more watts. A 30 lb power amp can push 1500w of clean power or more easily ( it's a FOH PA amp). So basically I have a 1200w svt2 rig
Those all tube 80lb+. heads will allow you to overdrive the power amp for some distortion / color if you want. I don't care for power amp distortion ( or preamp distortion) with bass. So the tube power amp section wasn't an issue for me.
Now you can use what ever preamp you like with any power amp pretty much, so say you want an ampeg tone , a demeter tone, a sansamp tone and what ever else. You can have any of those you like running into a power amp and basically change amps on the fly if you have a switcher, and keep the same power amp. Power amps don't usually ( or shouldn't) color the sound much so you can pick your power level and not worry about needing a new head because the one you bought isn't loud enough, or starts to distort when you turn it up.
This is my first component rig and honestly I don't think i'll go back to a head. I can use multiple main outs from my preamp for what ever i want ( tuner , FOH, a second power amp etc.) and can run pretty much any cab config I chose ( 2 channels stereo @600w @4 ohms, or 1200w mono) 2 4 ohm cabs, 4 8 ohm cabs, or 1200w into 4 ohms (1 4 ohm cab or 2 8 ohm cabs)
since the power amp doesnt color the sound much there are multiple choices at all different power levels. They have new class D amps out that push 1000-3000w and only weigh 7 lbs also ( and go for $200 or less). Thats what i'm looking at next. My rig ( preamp, power conditioner, tuner, power amp, road case) weighs about 50 lbs. I'd like to get that down to 25-30. The class D amps sound a little different to me, but live it doesn't matter much. My back will thank me later.