Help! Peavey 2x10 + 1x18 issues..

[D____]

Hello! Technical simpleton here, bit with a moderate degree of common sense..

Bought some cabs off a basschat user and plugged everything in last night.. Have an issue though and would very much appreciate any advice or cyber-diagnosis please!

The cabs are;

Peavey 2x10" 4 ohms / 175w
Peavey 1x18" 4 ohms / 300w

I'm using a Peavey Mk VI head, rated at 400w @ 2ohms.

The cabs are connected in parallel. I did some googling and snooping before buying the cabs to make sure the head would cope, and the general consensus was that it'd be fine albeit slightly underpowered. I used a few overdrive pedals and Tge setup is just for a small practice room at present so this shouldn't have been an issue..

So, plugged everything in. With both cabs hooked up, the sound is very weak and distorted. Unhooked the 1x18" and the full force of the 2x10" kicked in. Hooked up just the 1x18" and the sound was barely audible and generally a scuzzy distorted whimper. So, questions..

1. Was I correct in assuming I could run both cabs at a slightly lower combined power?
2. Is there any obvious reason as to why the 1x18" is being pathetic?
3. Does it just sound like the 1x18" has blown?
4. Could the damage have been caused in transit?

As I say, I've a reasonable level of sense in these things, but figure if anyone is going to be in a better position to put me straight, it's going to be you guys

Thanks in advance for any help!

-Daniel