First, yes, the cabinet makes the amp a complete thing. If you get the head, spring for the cab, if possible. It’s worth it, plus the cab is light (pine, fingerjointed) and very lovely on top of how good it sounds.
Second, when you do play the amp, be aware that because of the ppi design, the gain and master volume controls are highly interactive. The amp sounds best with the master opened up, or if the master is lower, dial up a little more gain. When I say highly interactive, I mean highly interactive. This is not a Mesa-style master volume amp, not even close. You have to work with it. But when you do, it rewards the effort.
The caveat is, it’s a pro player’s amp, and it helps to understand what it’s intended to be used for, in order to get the most out of it. If you get one, you may scratch your head and wonder what it’s about, but once you grok it and use it in a session or at a gig, you’ll get a better idea of its potential.
When I met Doug Sewell, who designed the amp, he and I talked about speakers, and he said he thought the Celestion Alnico Cream 90 might be really interesting with the cab. I’ve thought about getting a second cab and loading it that way, but guitar buying keeps stepping on that plan!