PXE-booting a Dell PowerEdge R6515 fails

I'm trying to PXE-boot a brand-new Dell PowerEdge R6515 in order to install 
NetBSD on it.

It's a long time sind I've done that (for our other servers), so maybe I'm 
doing something stupidly wrong.

The NetBSD PXE boot loads and runs, it then seems to load the kernel 
(which is a netbsd-INSTALL.gz), the cursor moves to beginning of the next 
line, and then the machine hangs hard.

The kernel is from 8_STABLE from around yesterday.

Any way to debug this or install via an alternate route?