@yyzsantabarbara how many positions did you try with your scenario? Positioning / phase / crossover / gain are all super critical in calibrating your subwoofer in your system. Also, are you using a high pass cutoff filter for your mains? I have found that cutting off below 80 hz on my mains (even large towers, the tekton double impact) helps the frequencies all the way up the chain for two reasons I theorize: one, the mains no longer have to push low end content so the drivers are doing less work, two, the crossovers and amplifier are also doing less work because the lowest octaves are now being handed off to a purpose built speaker (the subwoofer or preferably subwoofers)
@fritzenheimer I think your answer is, try with both. If you can use a cutoff frequency for your mains, that will take stress off your amplifier and your Kefs. I found this does wonders for my system but YMMV. If you find it's too much bass with the subs, try just adjusting the gain down or crossover so it's just filling out the bottom octave if your mains don't allow for high pass cutoff on the DAC or preamp. Sounds like you're about to have a rockin system. Don't get new subs unless you are finding the KC62s are not doing what you want