Low volume and unbalanced deck

Since the turntable itself doesn't have a volume control the problem is with the amp it is connected to. It should be connected to an amp with a dedicated phono socket. It's no use connecting it to a tape or aux or CD socket. These sockets do not have a built in pre-amp for the magnetic cartridge. If your amp doesn't have a dedicated phono socket then you can buy a Magnetic Cartridge Pre-amp that will plug into a tape socket etc on the amp and have the deck plugged into it. If plugged into a none dedicated socket though you can hear it, the volume will be very low compared to other sources. If you have got it plugged into a dedicated phono socket and the volume is still low, then the internal built pre-amp inside your amp for the deck has failed and will need to be repaired. The pre-amp failure will not effect any other functions of the amp.

Do you have these turntables on an anti-static mat, or some other non-conductive surface?

Turntable leads go to Phono.
Turntable ground lead (thin black wire) goes to screw terminal next to 'Phono'

CD phono cables go (unsurprisingly) to the CD

Cassette deck "Play Out" cables go to 'Tape Playback' sockets.
Cassette deck "Rec In" cables go to 'Tape Rec Out' sockets.

EQ Line Out cables go to amp's VCR/EQ Playback
EQ Line In cables go to amp's VCR/EQ Record Out

You should check the connections to the turntable, it sounds like it might be feedback hum, caused by a wire touching or something like that. Don't forget to check the connections to the cartridge too.

It could be a bad connection of the earth pins to the cartridge. One turntable I had hummed even with an earth connection. I found out that the cause was the connection of the earth wire in a 3 pin plug.

Problem could be in either the record or playback circuits of unit. Try playing back a tape in another machine to see what the volume is like. If also low then the issue is with the record amp circuits.
Fixing will not be cost effective, however.

Hi, are you inputing your decks through channel 3 and 4 phono?

Either the cartridge has failed, or an internal amp in the record deck has, or despite what you say the receiver has. I will clarify this for you. Magnetic cartridges need a pre-amp before it gets amplified by the pre-amp and power amp of your reciever. If that pre-amp fails you will still hear the low sound from the deck, but all others cd/tape would be normal as they don't go into the deck pre-amp.
All you need to is trace the circuit. Find the magnetic cartridge pre-amp and find where it joins with the pre-amp of the amp! If you stick a screwdriver at that point, you will get a buzz at normal volume on each channel. The cartridge pre-amp has gone if you do get a buzz!
You probably find it's a single IC that's gone, some are prone to static problems.

Those are just standard phonograph player characteristics in my experience. You'll hear a tap on the deck because there's essentially a live mic ( the needle ) physically coupled with the record on the turntable and the rest of the phonograph player even though they try to dampen the coupling of the turntable as much as possible. The rumbling noise at high gain is because the turntables being driven by a motor via gears or a belt and creates some  vibration, wich is transmitted through the turntable and picked up by the transducer ( needle ), low frequency rumble, just like a recording studio on a busy street may pick up from road vibration/ noise.

If you receiver doesn't have phono inputs,you will need to buy an inline phono amplifier.Phono inputs have a built in amplifier,all others don't.Try Crutchfield.com.If they don't carry them,they will tell you who does.