Amp trouble!

I went down to a local guitar shop today and made a trade of my old crate amp and an old fender and got a used crate half-stack... It has a VC-50H head, and VC-410E cab. I load it up, turn it on, and let it set and warm up for about 5 minutes since it's all tube. The problem is.. I can't get any sound to come out of it unless I turn standby on. After I do this, if I start playing it, after a little while it will kinda cut-out and get louder and distorted. It will do this on and off randomly. I dont know what the problem is... The guy at the guitar shop said the input might need replaced cause it gets bothered by the vibrations from the cab... but I have been playing it real real quiet. I dont know if I should just take it back and get my money on Monday or what. Has anyone here ever had this problem or heard of it? Thanks for any help at all.. -Mike

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PS.
Here are reviews etc on the head... I couldn't find anything on the cab...

http://harmony-central.com/Guitar/Data/Crate/VC50H-01.html

Tube amp power supplies are split into two sections: one low-voltage part for the heater filaments, and another high-voltage section for the rest of the tube connections. You should always let the amp warm up with the Standby switch turned OFF. This allows the heater filaments to get the tubes up to proper operating temperature before you apply the high voltage to the plates and grids.

When you turn on the Standby switch, the high-voltage part of the power supply is connected to the tubes. You won't get any sound out of the amp while the Standby switch is OFF, because the tubes don't work unless they have the high voltage connected. That's what the switch is for - letting the tube heaters stay on while the amp is sitting idle (standing by) without applying the high voltage to the rest of the tube connections.

Turning on the Standby switch before the tubes have had enough time to warm up will destroy them.

Is 5 minutes enough time for the tubes to get all warmed up? There 8 tubes all together (12ax7-type).

Yes, 5 minutes is plenty. If that's not the problem, I think it's time to have a tech look it over. And don't depend on a salesman for technical diagnosis over the phone. Bring it in to the shop.

Check around your input jack, there should be a washer or spacer around it so the input jack doesn't touch anything. On one of the 2 inputs on my Fender there is no washer so I am careful not to move the cable around.