Warm Interconnect....

with good details and treble extension (RCA). Is there such a thing. Budget is 300.00 and okay with finding some used. Thanks.
Cardas is widely considered to be on the warm side. I would probably ONLY consider buying used with cables.
DiMarzio M-path, $160 for 1.0M pair new. From headphone.com
I use it with my Nak deck, it is almost as good and in some ways at least just as good as my Purist Audio that cost five times more when new.
Nordost Heimdal. Not at all cold. Good dynamics and detail, with some sweetness in the mix. 1/2 price now at Music Direct I think. I have two pair myself and been using them for a couple years.
I've borrowed or owned about half a dozen IC's and out of those the Harmonic Technology Truthlinks (I own two pair) are clearly the most full bodied and warm and still had extended highs. That's not a huge sampling of what's available out there but might be of some help if the Truthlinks were on your list.
the quest for a warm cable has appeared on other threads.

the problem with achieving warmth, detail and treble extension is that the very nature of warmth denotes treble attenuation and and a slight peak in the lower frequencies. since warmth is a coloration , its presence will affec resolution. so, if you want warmth, you will get a reduction in clarity and treble response (spl).
Agree with Ssglx, heimdall all the way. You can read my older posts about it, simply amazing cable - it gives very, very appealing midrange bloom,, and still maintains enough treble & everything else..

Read the post and went straight to musicdirect, they still have the 50% off deal!! 349 $ for 1 meter pair is a even lower than a used on a'gon! believe me , when they will be out of stock, you will sell for at least the same price, when you will be going up the nordost ladder..

damn, I think I need to reserve one pair for myself..
Nordost CAN sound bright in some systems. You'd better audition these with yours before purchase. I recommend Purist cables for what you want.
If you go for used Purist with fluid not ferrox, make sure it is at least 2/3 full. Purist can refil for $100 plus shipping, I did it with my interconnects and speaker cables. Don't buy Museus and older Elementa - not good enough. As I said, try DiMarzio first, Purist are expensive.