integrated amp - jumper between preamp and amp

Anything you think you hear would be a placebo effect.  Think of it this way, those jumpers are quite a large gauge of metal, connecting the preamp and the amp boards inside.  They are much larger than the traces on the circuit boards they are connecting.  Essentially, the 2 small traces on those boards were moved outside the chassis and severed.  Then those large u-shaped jumpers were installed to connect them.  Those jumpers are also much, much larger than a RCA jumper that you could install there.  The wire gauge inside most all RCA's is tiny.  Possibly smaller than the traces on the boards.

If you do change them, before doing so tag them, L-R channel, jumper placement and direction of flow, generally perceiceived left to right should it be needed, that way, when you put them back they sound right.

A 125mm cable should be enough to know if an improvement is available, I found a pair on Aliexpress for $10, which would do the job

Interesting topic, I would have thought not.  However different cables between my individual preamp and power amp do make a difference in the sound quality. Out of 4 or 5 cables I have tried there is a clear winner among them that gives the best sound.  
Maybe worth a try for those with jumper included integrated amps.  

easythestrejamer said,  If you have a pair of RCA interconnects, plug them in to see if you notice anything. If you like what you hear, then you can consider buying a shorter "jumper" set.

Precisely! 

One of my first Audiophile Law articles I wrote for Dagogo.com, where I have been a reviewer for 20 years, was of the Dussun V8i Integrated amp, which had swappable jumpers. My initial experiment was to try interconnects. Then I had Jena Labs send a pair of their "Dussy" jumpers to test. Both passed my Law of Efficacy. I have written about that topic in my book The Audiophile Laws. 

I replaced the stock jumpers on my NAD C352 with customer Signal Cable jumpers and it made an audible difference. Experience is the best teacher. YMMV.