Sub Pairing With Harbeth 40.3 XDs

How about the B+W PV1D?

PV1D

A unique take on subwoofer design, PV1D uses two opposed drivers in a super-rigid enclosure to deliver fast, clean, firm solid bass 

VANDERSTEEN SUB 3

https://www.vandersteen.com/categories/subwoofers

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...