#1
What is up dudes? I am clueless this stuff so I'll try to give as much info as possible and hopefully someone out there can help me out.

I have a Peavey Mark IV 800w head and a Peavey Black Widow 2x10" 1x18" (think they call it the 1810 cab). Recently I picked up a Hartke 2.5 xl cab for pretty cheap and the other day I decided to pair it with the Peavey stuff. I hooked every thing up and it sounded badass! I was playing with a drummer, guitar player, and keyboard player in a decent size room. I was playing loud and clear over everything at fairly low volume. I didn't really push it because I was worried about what would happen and I really didn't need any more volume.

Anyway dudes, I just want to know if it is safe to keep playing with this configuration. Because, I don't really know much about wattage ratings or ohms or whatever. I don't want to blow anything up (even though the entire rig is only worth like 400 bucks).
#2
It should be fine assuming those cabs are either 4 or 8 ohm. The head is capable of running with a 2 ohm load.
#3
800w? Are you sure? Check the power rating on the cab, that should tell you how many watts should go through it.
Gear:

Guitars
Martin DSR
Taylor 214e
Fender Strat
Epiphone 335 Dot
Carlo Robelli Acoustic

Amps+Effects
1978 Fender Pro Reverb
Boss Blues Driver
Boss Super Overdrive
EHX Holy Grail
Boss DD-20 Giga Delay
#4
Yeah I think I am wrong about that. It might be 400. Shows how much I know about my own gear.