The manual (I have an A-S501, 301 is also listed) doesn't give input crosstalk figures, but (input shorted) phono s/n ratio is 82 dB OR MORE, CD on Pure Direct is 99dB or more. Crosstalk from other inputs will be considered "noise", and given the above are input shorted numbers, the actual crosstalk noise may be a bit higher. But in normal listening, any input signal will make the crosstalk imperceptible.
I doubt any reasonably priced amp will do much better, so don't feel this is a major defect or a reason to buy a more expensive amp. If the A-S301 does everything else you need, it is fine.
As has been pointed out, what you have is Cross-Talk. But to determine whether that is a problem or not, you have to ask yourself when what the last time you were driving TWO Inputs at the same time. For example Turntable and CD Player both playing at the same time ...and... if the very unlikely event that you did that, did you hear the Cross-Talk?
But, I would test it out with a Cable before I spend any money on RCA CAPS. Just to make sure it has an effect.
But, in general, unless you frequently play TWO Sources are once, the Cross-Talk is not going to be a problem.
The problem ..if indeed it needs to be considered as such is capacitive crosstalk between traces on the PCB inside the amplifier.
The signal from the cd player is 1v in amplitude ,and is being picked up by the other channels. The switching circuit is typically an IC and the traces are very close together. So a very large voltage induces a minute signal in the input adjacent. Since this will not happen in a real situation it is of no importance
I would be concerned if the amp was picking up crosstalk or other signals from sources not connected to the amp. particularly 60Hz hum.
Ok, some seat-of-the-pants testing. Working on the assumption (strong) that the OP's A-S301 and my 501 basically share the same preamp/switching section, I tried to replicate the OP's situation.
1) Line level music signal in the CD input, listen to Line 1 with no connection of any type for crosstalk. Yes, at 3/4 volume control setting and up, but barely audible over accompanying hiss.
2) Shorted the left Line input, all the same as in 1). Zero crosstalk or hiss on the left speaker all the way to WFO volume. Right channel remains same as in 1).
So depending on how efficient the OP's speakers are, how big the room is, there may be some slight crosstalk introduced IF multiple inputs are concurrently run at full line level and the volume control is at 3/4 or higher. But that crosstalk will still only contribute at the barely-audible dB level, compared to the probable 90dB+ actual program signal speaker output. Like trying to hear a whisper from your friend while you stand at the end of the runway with a 747 taking off over your head.
Solution: As suggested, shut off all input signals other than the one being listened to (i.e. don't have the CD player and/or turntable "playing" while listening to the TV signal), and there will be no other signal to crosstalk.
There is still some residual hiss when an input device (CD, Line, phono) is connected and selected with no music/signal being run, but it is less than the hiss from a totally disconnected line input.
So the OP's A-S301 is behaving exactly the same as my 501. No real-world fault, just a minor "problem" that MIGHT be solved with applications of large piles of cash.
How about some of our high-end residents doing the same routine as I did with their systems? Is this crosstalk/hiss "problem" ever reduced to zero on unshorted inputs of top-level systems? Where on the "cost scale" does this tend to disappear?
One more thing to add: When I went back to my "normal" input arrangement, with my Apple TV connected via the 501's Optical input, at 100% volume there was ZERO crosstalk and significantly less hiss when listening to any unconnected Line input.
So obviously the Yamaha Optical circuitry side-steps the first stage preamp circuit sections where line-level inputs are susceptible to crosstalk. If the crosstalk was from the input switching section or the tone section, it would still be there when the Optical input is used.