Best RCA interconnect up to ~$100?

I think you need to post your system to determine whether a cable upgrade is worth it. That money may be better spent elsewhere.

My vote is for World’s Best which uses Mogami or build your own from pure silver, shielded, balanced cables available from Parts Connexion.

They will become your actual reference cables from which all other fancy cables change the music one way or another.

From my DAC to my integrated I use the Parts Connection (DH Labs) cables, but from my HT processor/receiver I use Worlds best. All the other cables I’ve used somehow move the tonal balance, but always in a subtractive way. 

My system is:

source: pc with matrix X-SPDIF 2 usb to i2s, also a streamer via coax

dac: Psaudio Directstream MK1

amps: conrad johnson cav45 s2 integrate or schiit aegir with freya s preamp

speakers: JBL 4309 on desk for nearfield listening

kef kc62 subwoofer

OP: 

Often fancy cables will darken the sound, which if your speakers are too bright will be a good thing, but in your case a similar effect can be had by adjusting the direction of the speakers. 

Always worth experimenting with the tweeters pointing behind and/or above your head.  You may find your personal sweet spot is NOT on a laser line with the tweets but somewhat below them.

Best,

E