unused inputs/outputs - safe to short?

Update on this;

Actually, tonight I tested if my inputs 2, 3 & 4 are active when I use
input 1 on the preamp. Put a cd on (input 1), turned the volume right
down, switched from input 1 to input 2 and got the shock of my life!
With the volume about 50%, I can actually quite audibly detect a thin,
metallic rendition of the source. Same for input 3, but input 4 was
noticeably softer - needed about 65% on the volume pot to reach the same
level. I don't even think shorting out input 2, 3 and 4 would help
because the music seems to be transferred to the other inputs from input
1 on the internal wiring of the amp.

Do you think this could be a defect one should try to correct in some
way?

As you noted, its called crosstalk and sometimes is given in the spec. Caps are not shorting plugs and vice-versa. Different manufacturers have different opinions on shorting inputs. Check w yours to make sure it won't cause any damage. Then let your ears decide.

I don't think shorting plugs (or caps) will have any effect on the effects of cross talk in your pre-amp. Now if your tuner, for example, is off and you are hearing music from a radio station, you might get some help from shorting plugs. Might. I have used shorting plugs in several of my units and have had minimal, if any, beneficial results. I'm not techie enuf to even begin to understand the benefits of cap's although I've read the product claims.

Don't think there is much you can do except turn off sources you are not using.

FWIW.

In the good old days preamps would short all the inputs that were not selected. The rotary selector switch had an additional deck to do this. Many preamps, even cheap ones like Heathkit, also had a level adjustment for each input so you could change sources without abrupt change of volume. Also, Loudness control, Balance control, scratch filters, tone controls...did I forget anything?