Yeah, I wouldn't rule out the 8200, at least not right away. Different tubes can make a huge difference as well as output modes. It might have sounded too tubey and mellow to sunneebear because it may have been in Triode mode and thus Ultra-Linear mode may be a better match, as it tends to be with most speakers. Sunneebear, do you remember what mode it was in at that time? Even if it was in Ultra-Linear mode, the source and other things may have been attributing to this as well. Either way, the 8200 gives you a ton of options to tweak the sound to your liking. If there is one comment I hear most from those that hear my 8200, especially from my buddy who has heard it many times, is that it isn't a crazy tubey sounding amp. Nothing like the Bottlehead Crack and the like, which while really seductive/smooth, tends to be too bloomy and way too slow for my tastes in general. My 8200 is just right for my ears and has the attack that other tube amps tend to lack. Of course, this too is just my opinion and I do have upgrades in it, so it isn't stock.
Regardless, they are both really great amps, although I unfortunately haven't been able to spend as much time with the newer 8600, especially compared to the time I've had with the 8200, since I have never owned a 8600 and have only heard it at RMAF twice. I will say, that after many years now, I still have no desire to go with any other amps, and I've tried a ton of different ones, including very expensive ones, since owning my 8200. That is one thing that is great about these amps, as well as other offerings from Elekit, for the price they simply cannot be beat in my experience.
My best recommendation, of course without having much experience with the 8600, would be the 8200, since it is cheaper, smaller, and doesn't require expensive 300Bs, etc. Actually, one of the big positives in my opinion of the 8200 is the vast amounts of different types of tubes you can use in it, and even though some are very hard to find and expensive, new production are available as well; just lots of options in the end. With all this said, if you plan to primarily use the amp to drive speakers, the 8600 would probably give you more of what you are wanting ultimately, but if like me, you will primarily use it with headphones and just occasionally or secondarily with speakers, than 8200 may make more sense or if just to get your feet wet within the world of Elekit initially.
Either way they are both great amps in my experience, but continue to let us know if you have questions!