Impact of low quality analog RCA interconnects

Smoe cables can work to reduce sibilance. All you need to do is search for warm sounding cable using the search feature of the forums.
That said the problem of sibilance is more likely a problem with your electronics and tweeter type. In paricular some SS amps have a harsh digitis that is particularly pronounced with some metal tweeters. There has been a lot written about that also.
Back to cables if you are going to upgrade from the free radio shack cables to anything well known for warm soud like certain Cardas cables be prepared for sticker shock. In other words most well regarded interconnects are pretty expensive. Even a modest pair of fairly neutral Audience brand Conductors, their least expensive model, has a list price of about $180 for a 1.5 Meter pair. I paid as much as $500 for a 2.0M pair of used Cardas Golden References a couple of years ago. That cable would have a chance at calming down your sibilance it is a warm type cable.

agree with Mechans, my experience is low quality interconnects cause closed in soundstage (car radio sound) and loss of detail.

Thank you for the advice.

I ordered some chord chameleonplus interconnects that are supposed to be a good match for the spendor s8e. I hope that this will solve it.

I will do a bit more research on the amp- tweeter issue.

From the feedback, it appears that the dac is unlikely to be the cause. Correct?

My spendors have a silk dome tweeter. Any issue with these and amps?

what other equipment do you have that you are driving these speakers with?