First, I would take "what others report" with a heavy dose of salt. I'm not a fan of the Big K, so can't really tell you whether there is significant bass in those boxes or not... but they are rated down to 32Hz (not true full-rangers) with an overinflated sensitivity rating of 102dB! So assuming the standard level of their ego, its probably closer to 93dB...
Either way, the Denon on its own should drive them to ear-bleeding and permanent hearing loss levels.
I digress.
32Hz is a pretty respectable extension for music. You are only really missing out on the lower keys of the piano and Organ... maybe some esoteric percussion... and electronic/manufactured music.
An amplifier won't fix that.
Your Sub will. (And looking at your room acoustics.)
Also, YouTube audio quality is spotty at best, and a couple users have run into situations of almost damaging their gear from very poor source quality. Please be careful.
I would run your Sub with your mains, and call it a day. That SVS is probably going to give you much better bass rolled at 80Hz, and allow your speakers some breathing room.
Cheers!