Sub Pairing With Harbeth 40.3 XDs

Some of these Subs like the REL S/812 seemed kinda pricey....an Emminence $250 speaker in a shiny powered cabinet doesn’t seem worth $3200?

Yes it's quite expensive but it delivers.  It's the performance that matters. Some costly hifi may not perform up to expectations and that's money wasted. I have a single S510 and it transformed the listening experience. The old sub that it replaced cost close to $3k MSRP back in 2009 and there's no comparison.

All good points. It’s hard to explain and I presume more knowledgeable or experienced folks would be able to describe it better. My old sub can be considered as poor as it is either too "in your face" or too subtle. If the bass sticks out too much, it’s unwelcome bass as the excessive bass which sticks out like a sore thumb is making more harm than good. If it’s too subtle, you can’t hear a difference. Good subs which are able to blend in well with the main speakers will make their presence felt, and the bass won’t feel like it’s detached or separate from the main speakers. In other words it’s a perfect seamless bass. A deeper, more defined and satisfying bass without anything sticking out too much like Home Theater the-bigger-the-better bass.