Many many subs have both RCA and speaker level inputs, agreed. The point is NOT TO connect speaker level outputs from the AVR to line level inputs on the sub. Only connect line level to line level, speaker level to speaker level.
Some Simple sums:
Error #1 - Connect Line Level (RCA) Output from the AVR to the Speaker Level Input on the Sub
The line level outputs of an amplifier are typically around 0.3 volts RMS. The impedances of the downstream devices are usually high (typically 10,000 Ohms or 10kOhm, so that very little current.is drawn through the line (RCA) wire.
Typically, the current driwn, I, is given by the voltange applied (V) divided by the impedance (R). That's Ohms Law as you may remember from school physics.
S the current drawn through a properly connected line level cable is 0.3 volts divided by 10,000 Ohms = 30 MICROamps.
Now, if you put 0.3 volts across a typical 8 ohm speaker, it'll try to draw 0.3 volts divided by 8 Ohms = 37.5 MILLIamps, or about 1,250 times more current.
This will almost certainly blow the line out circuit on the AVR, as my teenage stepson found out the hard way.
Error #2 - Connecting Speaker Level Outputs to Line Level Inputs
Now. let's assume you're trying to pump, say 32 watts (not a lot) into an 8 Ohm speaker. 32 watts keeps the sums easy, which is why I've chosen that number. The power in Watts is given by the current in amps squared mutiplied by the impedeance. So the current is equal to the square root of (32 amps divided by 8 Ohms) = the square root of 4 = 2 amps. See I told you 32 watts kept the sums easy!
So. 2 Amps, 8 Ohms, That, using Ohms Law again, means a voltage of 16 volts. So what your AVR amplifier does is to raise the voltage to 16 volts and the current that flows is a consequence of the voltage, not the other way around.
So you try putting 16 volts across a 10,000 Ohm line level input. That's 16 volts comapred to 0.3 volts, or about 43 times as high as the line level voltage it was intended to handle. The resulting current will be 16 volts divided by 10,000 Ohms = 1.6 milliamps, or about 500 times what it was intended to take. That'll sure as Sherlock fry the low voltage side of the sub's internal amplifer, and it it doesn't, it'll be tring to push so much power through the sub's speaker coil it'll blow that instead.
If you don't believe me, try it and get back to me. After the Fire Brigade leave.