Speaker cable recommendation please

Components:

Audio Research SP5 Pre
Audionics CC2 Amp
Klipsch Heresy Speakers

The current speaker wire being used is the small diameter original Monster Cable in 25 foot lengths. The spade terminations were done in the mid 80's and there is obvious oxidation along the length of the cable.

I have been asked to make a choice for a life long friend in this regard. Here is what I'm up against. This friend is a 52 year old avid hunter/trap/skeet shooter and has suffered some hearing loss. I can't swap ears with him but my sense is that he simply needs more gain to hear what others hear.

The Heresy's have the upgrade woofers but are still bass shy, in part due to the room size (36'x22'). Cost is a factor ($500 budget) and moving equipment to shorten the run is not an option because of direct sunlight/wood burning stove considerations. The speakers are placed close to the rear wall, on either side of a large window but not close to corners.

Whatever you guys suggest should hopefully increase bass output and must not make the system any brighter than it already is. I may be able to spend more money than budgeted and add a subwoofer. If the concensus is the addition of a sub please advise what would work well with the Heresy's and keep in mind that I would still need to upgrade the existing speaker cables due to their obvious deterioration.

I would like to thank all you guys in advance for sharing your knowledge. I'm approaching my one year anniversary of finding Audiogon and appreciate all that I have learned from this great community.

Regards,
Patrick