PMC EB1i v. Thiel CS3.7 v. Pioneer S1-EX v. ???

Since you have PMC on your list I would suggest that you look into active speakers from ATC.
Large 3-way active PMCs (not just activated) are out of your price range, but active ATC 50/100 could be found used for that $.
Reason I am mentioning this is that I have owned and/or auditioned many of PMCs and ATCs in both active and passive version and active ones always sound considerably better, without exception.

I have owned IB2S and a number of smaller PMCs, and extensively auditioned MB2S, EB1, EB1i, IB2i, PB1i. There was a whole range of other speakers as well, bunch of models from well known manufacturers such as Wilsons (Max 2, W/P 7 and 8, Sasha, Sophia 1 and 2), Revels (Salon 2), Harbeth (Monitor 40.1), B&W (804s, 802d), Energy (almost entire range), PSB, JBL, the list is quite long, as well as some less known but promising manufacturers.
The PMC own midrange driver in IB2 and upper models is certainly better than what appears to be Vifa in lower models.
Each pair of speakers had some issues however.
I found that passive PMCs do sound better to me than passive ATCs, passive cross-over in PMC appears to be better design as it produces less integration issues, but active ATCs sound better than anything passive from PMC or ATC in corresponding range.
I have left the world of passive speakers, benefits are substantial.
So my order of preference in passive PMC arena is MB2S, IB2S, IB2i, EB1i, PB1i.
You will notice that I placed IB2S (the “professional” model) above IB2i (the “consumer” model), reason being that I consider PMC midrange driver the main benefit in lineup, much more so than the new SEAS tweeter found in “i” series, and I pay no attention to finish.
In other words, I would rather go up in “professional” lineup for the same money than going into “consumer” lineup, if you catch my drift.
This does not mean that SEAS tweeter is not different or would not be “better” to you, you would have to decide.

I cannot offer at this time a meaningful comparison of active PMC to active ATC, but I will be able to do so in a month or so when I will have them side by side.
I owned IB2S and have auditioned IB2i, and I agree that IB2i sounds incredible, but active ATC50 is overall more incredible to me, considering that there is no perfect speaker and all have some shortcomings.
So far my preference has been passive PMC over passive ATC, and active ATC over passive PMC, overall, based on models I have heard.
I find active ATC to be better than Wilson Sasha and Revel Salon 2, I also prefer passive IB2S or IB2i over these two, but I must say that Revel Salon 2 is also extremely well made speaker with very good integration of drivers, and I prefer Revel Salon 2 over Sasha considerably. About the only thing I find objectionable with Revel Salon 2 is a slight metallic coloration which I guess is the consequence of materials used. And I prefer Revel Salon 2 over lower PMC models.
I agree with Ryder’s assessment of Harbeth in comparison to PMC.
Finally, you will notice that I speak of my preferences, your ears should ultimately guide you.

I prefer ATC SCM50ASL and larger models to passive IB2S.
Some may prefer an apparent low end extension of IB2S, but that comes at price. Active ATCs give you better window into the recording, the line of hearing things never heard before is what comes to mind, but not through hyped up presence region or higher frequencies, there is a clear separation of instruments, even when they occupy space near each other, individual drivers never calling attention to themselves, I am convinced all being the product of superb integration of drivers not being possible with passive cross-over. On passive speakers you simply do not hear what you do on active ones or on headphones.

Ssolman, what you described with the track from Alison Krauss is what I found generally with all good recordings, you keep increasing volume and it makes you want more of it.
I have not yet received active IB2S-a, they are on their way.
Note that I will have them side by side with active ATCs, not passive IB2S or IB2i, but I am very familiar with passive ones since I owned them.

Yes, amplification is Class D, I am not certain but I believe it is Hypex. The choice was probably driven by space, low heat generation and power consumption. It is quite possible that digital cross-over is also Hypex job, but it is only my somewhat educated guess. I believe it is beneficial to have TL on the opposite side of LF driver and this was part of the redesign. The oval shaped grill is there to minimize diffraction. All in all it seems well thought out design.

In regard to active PMCs not showing up on used market while ATCs do, it is understandable, truly active PMCs are huge and very expensive beasts, this new active IB2S is PMCs smallest and first 3 way truly active (not just powered, or “activated” as they call them) in domestically manageable size. On the other hand ATC has quite a nice lineup of actives, from domestically manageable sizes up to their beasties. So I am not surprised you see more of active ATCs on Audiogon. On the other hand you have many more passive PMCs for sale for obvious reasons.