So I've got a Randall RH50T all tube head, going through a low end Ibanez cab. Most people will probably say get the new cab first, but I'm not sure.
The sound I've got now, I'm very happy with, but feel it could be better.

I'll have around $650-800 to spend.

Head wise, I was thinking of a Krank Rev JR. (50 watt one), a 5150/6505, or a Mesa Single Recto, but I'm open to suggestions.

Cab-wise, I was thinking a Marshall 1960, Mesa Recto 4x12, or an Orange 4x12, and I'm also open for suggestions.

And finally, I'm playing an Ibanez S520EX with EMG's, 81(b) and 85(n). Also in use is a Digitech Bad Monkey.
The style of music I'm going to be playing is along the lines of Suicide Silence, Sea of Treachery, As Blood Runs Black, The Acacia Strain, yadda yadda yadda, I think you get the picture.

Sooo, head or cab (I'm guessing cab), and any suggestions? Thank ya

(This is mainly just a matter of me not being able to pick for myself, I believe the cab would be better but I definitely wouldn't mind a new head. This is just for reinforcement before I make a final decision).
I'd go with the Marshall cab. It ought to be a nice blend with the Randall.
-Gibson Les Paul Studio Cherry Sunburst w/ Alnico Pro II
-1960 Yamaha Orange Sticker Acoustic
-Marshall JCM2000 DSL 401
-Fender Hot Rod Deluxe
-Blackstar HT-5RH
Well I guess more important is are you happy with your amp head tone? does your distortion channel do the trick. I see your using a bad moneky which I'm guessing is similar to a tube screamer for boost mostly. If you are happy with the head then yeah go cab and experiment with the cabs at your guitar store. if not then get the head.

When I got my Valve King I brought my cab to see what it would sound like compared to the matched cab and I liked my cab better than the peavey sound honestly and I'm running a low end B-52 cab.
I'd just reload the cabinet you have. Cheaper, and then you can get a decent overdrive or delay or some other toy?
"If you're looking for me,
you better check under the sea,
because that's where you'll find me..."