Sub Pairing With Harbeth 40.3 XDs

For my 30.2's I got a JL Audio F110v2. It mates nicely with the Harbeth sound, can't even tell it's on or where it is. Just sounds like the 30.2's with a bit more bass. So that's probably the biggest trick, matching to the Harbeth voicing. I had a Volodyne that did not sound good with the Harbeth, it was a good sub, it just didn't blend well. 

But there are plenty of good subs out there, it's just that this match worked out well for me.  Wish you luck on your search. I'd love to know what you end up with.

many would say mon 40’s do not need subwoofer bass support, but get a pair of rel’s if you must

Why would you need a sub with those Harbeths? They should sound good just as is...

needing a sub and wanting a sub are two separate camps…I initially tried it just for fun and found adding a sub to be a total game changer. 

Have you called Harbeth or visited a local retailer to listen?  I personally like REL because of their high level connection.  I makes sense because they act like woofers if dialed in right.