Impact of low quality analog RCA interconnects

There's really only one way to find out... sibilance could come from your source before DAC. For interconnects, I recommend either Blue Jeans LC-1, canare L-4E6S, or Signal Cable Analog One or TWO.
With these, you are sure to get really good cables at the right price. Of course, you could spend hundreds more for better (hum) or for worse, i.e. AQ, Kimber, ZU, or iBELIEVE xr26gold with super secret B3 technics.

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?

Hashy-sounding sibilants are the result of poor transmission of the original event. It can happen anywhere in the signal chain from the microphone chosen to the recording itself or the digitization in the studio, or on playback it could be the DAC or the signal chain thereafter.

Cables with a rolled off treble reduce the amplitude of harsh sibilants, but don't fix the problem. Better interconnects with very fast rise time and extended bandwidth may help, as they resolve the smaller parts of already small wavelengths (sibilants). Of course they can't fix things that might be upstream. But even there, faster components with better resolution will either reduce or fix the hashiness depending on how much of it is encoded in the recording itself.

Whatever else happens, you will notice an improvement in your music with your new Chord interconnects. Good audio is--among other things--dependent on good signal transfer. Cables are passive devices and each cable run takes a little something out of the music, some more than others. Better interconnects have wider bandwidth, more conductor material, fewer impurities, and often better impedance matching for the components involved.

I've never heard of a textile dome tweeter being a problematic load for amplifiers. I don't know the specifics of their general impedance curves but they have been used for many years in speakers that match well with receivers and tube amps that can't handle tricky loads.