Sorry that I answered a question different than the one that the OP asked. But hey, this is Audiogon, if you know what I mean.
Re: Harbeth's frequent updates, I don't see anything sinister or manipulative about them. The company culture entails constantly fine-tuning its products and every once in a while it releases an interim upgrade. I of course have not heard every version of every model, but in general, I believe that each update is a second-order improvement. That is, every iteration is, foremost, instantly recognizable as a Harbeth box. The upgrades, usually involving internal cabling, crossover components, or driver materials, generally deliver the kind of sonic improvements that require direct A-B comparisions for most people to hear.
So I'd wager that the difference between the M.30 XD and XD2 is no greater than the difference between the M30 XD2 and a C7ES-3 XD2 (other than the aforementioned bass extension & dynamics).
Thus, IMHO, the decision whether to upgrade is a toss-up. If you're unhappy with your XD, or consider it the weakest link in your system, OK. But if you like what you hear now, maybe the cost and effort may be too much for a relatively tiny sonic improvement. But only you can make that decision. If you can't audition the XD & XD2 side by side, it's a coin toss, based on the limited information and personal opinions you'll get here & from TAS/Stereophile.
But I bet you've already gathered that! Audiogon is a great place to chat and make friends (so long as the trolls don't come out). But as an information source upon which to make mulikilobuck decisions -- not always so much.