I misspoke earlier because I forgot you run tube gear. Your tube amp is relatively high impedance compared to a solid state amp and your speakers are low impedance. It's a really bad combo and any improvements in the cable, will make a big difference.
The fancy cable is not only 1/6 of the DC resistance of the Belden, its equivalent capacitance and inductance is also lower. That will make a difference in the highs of your system.
How big of a difference does the fancy cable make over your Belden? How about 1dB big for voltage seen at your speaker terminals? That's significant and it's about 10% of the voltage sourced from your tube amp. If the amp was pumping out 50 Watts, only 40W was making it to your speakers!
Now I really do recommend you try 8 AWG cable and save some bucks.
