I also find the RCA jacks on the SB3 to offer a tighter-than-normal fit.
However, I believe you will discover that your DH Labs Air Matrix RCA interconnects use RCA connectors with locking barrels. As Cruz123 suggests, loosen the locking barrels by turning them counter clockwise. Install them, and then tighten down the barrels to a snug, but not overly tight fit.
However, I believe you will discover that your DH Labs Air Matrix RCA interconnects use RCA connectors with locking barrels. As Cruz123 suggests, loosen the locking barrels by turning them counter clockwise. Install them, and then tighten down the barrels to a snug, but not overly tight fit.

