suggestons on speaker cable that boost mid and upper bass

YW. Within yur budget, you could pick up a pair of either single wire or biwire Cardas Audio Clear Beyond cables around $4500. Those cables won Stereo Mojo’s ’Maximum Mojo’ award & should give you the qualities you’re after. This biwire pair are new and the guy is also offering free insured shipping in the US & will carry the Paypal fee - https://app.audiogon.com/listings/speaker-cardas-audio-clear-beyond-2-5m-speaker-cable-bi-wire-new-w... I did see another pair of single wire versions in mint condition with free shipping also.

Since you are happy with the highs and design of the Connex, I suggest you research ICs that have the characteristics you seek. Don't dismiss what a change in IC's can add. It could fill in the mid and bass.
Possibly the Cardas Clear Reflection.

bigdish: Cable cannot "boost" mid and high frequencies. They can't add something to the signal. They can, however, REDUCE the bass giving the illusion that mids and highs are more prominent. Best to think in these terms.

Going from a multi strand silver cable to a single, solid core copper cable of high purity might be the way to go. It's been my (limited) experience that even well made multi strand cables impart a lush and pleasing midrange with a lessening of the tone, texture and detail in the lower bass. You can't have it all. Accentuate one aspect of the frequency and other parts can, and will suffer. Balance is the key.

Single, solid core cables seem to do the least harm to the signal and the end result might be what you're looking for.

Also, what lowrider57 says is true. I'd even go so far as to say that the serious coin should be spent on the ICs and once you have that dialed in, the speaker cables would be easier to sort out, provided that you're ICs are up to snuff. 

All the best,
Nonoise

Thank you all for your suggestions. Mich appreciated.  For ICs, I am using Stage III Concepts Gryphone from SCAD to preamp and PAD 25th Anniverary from preamp to amp. Both are great ICs. Thats why I think my speaker cables are the weakest link. I am also thinking of copper wires. Anyone has experiences in PAD and Kubala Sosna speaker cables? I heard they have great density and flow and are a bit warm. Interested in PAD Neptune or Kubala Sonsna Emotion or even Elation. A pair of Dominus are just too much when buy new. Been looking for second hand pairs but not available at the moment. How does Cardas Clear Beyond sounds?