interconnect suggests under 300 used.

I have tried many different cables over the years. I have come to the decision that Synergistic Research Cables are the BEST Sounding, It'll Bring the Best out in your system for the Money! Now that SR has ACTIVE SHIELDING it takes their Cables to a New Level ( World Class ). I'll compare them to Cables at least 2x the Price! All I use in My system now is SR ACTIVE Shielding Interconnects,Speaker Cables,and Power Couplers. Go to : synergisticresearch.com
You can Probably pick up a Good Pair here on AUDIOGON. Good Luck and Enjoy!

Good suggestions Ivanj. I usually like to work the other way around. A delicate sounding IC for between the CD and preamp - I find it better preserves the low level signal - followed by a more robust cable between the preamp and amp.

In my cable auditions I found that the second cable (between preamp and amp) was more difficult to finalize because the robust cables tend to have a richer sonic signature.

What three cables? Considering you have ML gear, I would work toward something like Nordost Quattro Fil between CD and Preamp and Cardas Golden Reference between Pre and Amp. That might sound nice. However on your budget... you might look at something like Audioquest Lapis or Nordost Red Dawn followed by Cardas Neutral or something from Tara Labs.

Lots of used cables on the market in this price range. I also own ES speakers and have found that a well designed pure silver cable sounds best (speed and resolution of low level detail). Brands I have tried are AZ, Silver Audio, and Pure Note. The Pure Note Sigma at $400/1m (new) may be a good match for your system.

I will increase the anti if the improved sound is worth the expense. I will up my budget to 600 for interconnects on the used market. Will this help?

Electrostaticman - it will most certainly help. I have NO doubt. The right interconnects (and power cords) bring about an amazing improvement in any system.

In fact, I have been hesitant to state this in "public" but cables can make more of a difference than any other component. I know its may seem strange strange but...its true. The old adage, "10% of your system price should be spent on cables..." is BS! Spend more!