I know a guy who knows a guy who might be able to hook me up with one of these for around $800. Good deal?

wait a minute... wasn't this head discontinued?

A little better than average if it is in excellent shape with healthy tubes.  An okay deal for awell used but functional head.  Not so great if there are any issues.

does mesa still make all tube bass amps?

the 400 and 400+ were the tube jobs, discontinued but still around.

A buddy of mine had a 400 15 years ago.  It sounded great but it was never reliable.

I've had my 400+ for 16 or so years, I have changed the tubs one time and the amp NEVER let me down, Yes it's big ,it's weight a ton but it's a monster at 4 or 2 ohms. I've seen them go for around $600
 
 
I love mine
Sir David

how about the M-Pulse 600?

If you are looking for all tube alternatives - we recently received the Demeter VTHF-300M Tube Power Amplifier. 300W of tube goodness. With a nice preamp this is the best amplifier that I have ever heard.
 
http://www.demeteramps.com/products/poweramplifiers/index.html

You could buy two 400+ amps and still have money left over for what you'd pay for the Demeter.  I'd look at the Sunn/Fender for a versatile amp at a reasonable price.  I'd also look at the Traynor for a nice tube tone in a size that doesn't break the back.  200 tubey watts are good enough for many situations.