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).
It should be fine assuming those cabs are either 4 or 8 ohm. The head is capable of running with a 2 ohm load.
800w? Are you sure? Check the power rating on the cab, that should tell you how many watts should go through it.

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

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