The 30.8 is a great amp and definitely has the most bass and most overall power of the "low" power Pass amps. I agree with the 6moons reviewer's thoughts that the XA30.8's sound is built from the bass region up, whereas other Pass/First Watt amps build their sound from the mids or even the highs to some extent (in the case of the F5). The 30.8's mids and highs are also really lovely, but it's the bass that really grabs one's attention with this amp. I liked it very much, but it dissipated just too much total heat for me, especially after my family and I moved and my listening room shrunk. But even in my larger space (prior to the move) it would easily contribute to a warmer overall room temperature, similar to the amount of heat another person in the room might generate. I also found it a little noisy for my Omegas, which I know is not a pairing that this amp would normally be used with. Note I also listen nearfield now (so noise is certainly a major factor for me). This probably is not a problem for most other speakers (with multiple drivers and passive crossovers), including Spatials. So I'm really not surprised you got great synergy loki7177. I bet it was a fantastic pairing! Spatials need space to breath as does the 30.8.
I liked the F7 quite a bit, but in my case the bass and lower mids were a bit over damped (with my speakers) which are easy to over damp. What some may find interesting is that I find the XA25 has a much fuller presentation despite having significantly more power (and overall control) than the F7. I had sort of the opposite experience in my setup in that the F7 sounded rather tonally flat, due to the lack of fullness, which I believe was caused by the over damping factor. Note : I know this thread is about pairing with Spatials (not Omegas), so please keep this in mind as you read. With this said, it makes me almost wonder if something was wrong with my F7. I doubt it, but I don't recall it running super hot as you describe...definitely not to where I couldn't keep my hand on it. I may just be less sensitive to the subjective "can I keep my hand on the amp" test.
I may have to give it another try some time with speakers that present a little tougher (more appropriate) load. The F7 runs pretty quiet though. I remember that much.
The SIT-3 (which I also currently own) is wonderful and I agree it is the most liquid. It doesn't have quite the top end extension compared to the other three amps, but the highs are indeed sweet. It definitely has a realism and presence factor I'm not quite sure I've heard from any other solid state amp. Imaging and image outlines can be down right spooky. It is special, for sure and I'm happy I got ahold of one. I might add that the SIT-3 is SUPER quiet too. To hear even the slightest hiss or buzz, my ear has to be within maybe 2 inches of my speaker drivers. Wow! The only problem is that this amp is not nearly as speaker load agnostic as the other amps listed. I think this is its only major consideration (well and total output of course). It is very YMMV.
Thanks for sharing your thoughts loki. I enjoy reading about the experiences of fellow Pass enthusiasts, especially those who have extensive experience. My thoughts closely match yours in most respects. It seems where we diverge a bit has much to do with our speakers and how the amps react to their very different loads. I can certainly see how the F7 may be a better amp than the XA25 in some cases, not just in terms of speaker matching, but in terms of simple listening preference. The XA25 is indeed quite neutral, and it will let you know if your speakers and front end are playing well together or may simply inform you more about the kind of sound or presentation you prefer. It is chameleon-like in that respect. Synergy is always key and what may look great on paper might not pan out so well in practice. But sometimes the opposite can be true too. So if one has the means, definitely try things out and see. I agree with you that the F7 is a great starting point (and perhaps ending point) among all Pass/First Watt choices. It is not the most expensive option while still being very versatile. I started with an M2 many years ago because it was the least expensive (current) Pass design I could get my hands on at the time. I'm very happy to have simply given it a try as it really opened my ears and helped inform all my gear choices after that point. It was a game changer for me.