Hmmm... it sounds like the hum you may have been hearing was possibly AC heater hum leaking into the preamp?  Did you check your heater wiring dress?

Indeed, there would be no hum with all volumes on zero if it was just preamp noise.  I'd look at the heater wiring.  Could also be one or more tube sockets of the 5 has the wiring reversed.  Even though that is AC and technically not polarized, you can introduce hum by not keeping things the same for all sockets.  Just a thought...  Maybe could be a heater wire too close to a grid wire, too.  Chopsticking will help you find that.

As far as how quiet the Chupacabra is when the volume and gains are all down, you won't know it's on (no BS!), except for the pilot light.  It's that quiet, I promise.  It has DC heaters, so it's super-low idle noise.  -edit-  I meant to add you will hear some gain hiss at really high settings, but that's normal for this amount of gain (a LOT).

If you want a Marshall, you will like this amp.  Greenbacks are a great fit for this amp, as well as Vin30s and T75s.  I'm sure G12-65s would be equally great, but I have not tried them yet.

How quiet is the Chupacabra 50?
Pretty quiet with the Era switch to the right.  Bedroom volume is no problem into a 4x12 cab.  It's not as quiet with the Era switch to the left and get pretty loud with the Era switch in the middle.  Of course, turning up louder makes it sound better, as with most amps.

Clips are sure to spring up pretty soon.

The Chupacabra does so much more than just the high gain '80s sound.  It also does a great modded plexi sound and with the right settings, a good Marshall clean sound.  It's been said there is a great modern metal sound in there too.  The EQ section is very flexible and there is a great range of gain on tap.  This is a monster amp, no doubt about it!

I have some clips here:
http://www.soundclick.com/bands/default.cfm?bandID=682660&content=music

Look for the EVH Tests and anything with the Load Box should be the Plexi Brown 100.  They aren't the greatest playing and I'm not that great at recording, either.  But it will give you an idea of what the amp sounds like.  It's a great amp and sounds better live than I can ever record it.

BTW, my PB100 has the split cathodes and a PPIMV installed.  Great amp!