Would there be any reason not to use a consumer 2-channel stereo receiver instead of a PA system amp?
I doubt the system ever gets pushed hard enough for wiring to be an issue. I wouldn't worry about it. That said, 12 ga zip cord is relatively cheap, under thirty bucks for a hundred foot spool, so upgrading is simple and inexpensive.
For the main amp you could go with a residential amp as a stop gap, but it's better long-term to get something more robust. Bedlamite mentioned Crown. They're always a solid choice. Check with your local music supply store and see if they have anything on the Used rack. There's always someone trading up.
In terms of your stated prob, I would look at your mixing board first. Make sure someone hasn't dorked with gains or messed around with volumes (both channel and master). Reseat all cables and makes sure it isn't a simple dirty connection.
What model is the Carvin. We might be able to come up with simple
in situ tests and nail down where the problem is. Parts Cannon repairs are usually unreliable and expensive. Let's see if we can pinpoint a source first.
When you say "volume control is scratchy and has dead spots", do you mean the mixing board or the amp? If the amp, why are you using it for volume? Once the main amp is set, it should remain undisturbed with volumes controlled at the mixing board.
One trick to try on scratchy pots is electrical cleaner spray and spin the heck out of it. Hose it down, then spin it stop to stop ten or fifteen times. That's solved many a problematic volume control.
Brad