Warm sounding cables for STAX setup

I'm looking to get some warm sounding cables for my STAX headphone setup. Right now it is:

Bel Canto DAC2 -> STAX SRM-313 amp -> STAX SR-404 headphones.

I'm very happy with this setup, but it could use a little warmth. As it is now the setup is tipped up in the treble, and a bit too forward in the upper midrange/lower treble.

I'm on a strict budget (student) so I don't want to spend too much. And the cables will be bought used.

For RCA interconnects I'm looking at Kimber Hero. Also wouldn't mind trying a power cord, but no idea what to look for.

Try Signacable Analog2 or Monster M1000 series
I can't suggest anything better than the Hero in your stated price range, so that might be the right choice. I would strongly suggest you be very careful in changing power cords, as it could move you away from your stated goal of added warmth. What power cord(s) are you using now?
Another very cheap way to try for the psycho-acoustic effect of warmth in the digital realm, herbiesaudiolab.com sells their Grungebuster2 CD mat for only $12.50. Works by reducing vibration/resonance at the source, counteracting what our ears can interpret as treble emphasis. Also, trying three Herbie's tenderfoot[ers] under transport, DAC, headphone amp may prove amazingly effective at bringing natural warmth to a digital playback system. Cheap, and only takes a few minutes of positioning to see where it works for you. Ninety day home trial offer is unique, and I doubt you'll mail them back to "Herbie".
Also, if you buy an older model of Marigo Apparition digital IC (now, cheap on the used Audiogon market) to connect between your (unidentified) CD transport, and your BelCanto DAC2 it won't break the budget, yet can excel at rejecting high frequency glare, and "in your face" upper midrange, lower treble. This manufacturer delivers pleasing detail, minus any unnatural highlighting within the audio spectrum, even in older versions of this classic digital IC line.
Analogous to the way a luxury limousine suspension irons out a bumpy highway, without losing road feel.