I’m a dealer - McIntosh, Parasound and Klipsch, Sonus Faber, etc.
I personally owned Forte III for about a year and during most of that period I drove them with a MA252. I honestly never loved the combo. It was nice to have a little EQ for the Forte III and that’s one place where the MA352 has more to offer. Not to mention the added power. But I find the overall presentation of the 252 and 352 to lean a little dry and less tube-like. Good with Sonus Faber, less so, IMO, with Klipsch.
Of note, while the Forte III was rated in the mid-to-high 90’s as far as efficiency, I found it to be a little misleading. Continue to read and you’ll find that I drove them with a McIntosh MC462 before I sold them and the difference in grip and speed was palpable. I’d really wonder if part of your issue with the Single-Ended, 300B 508IA may be that the Forte just wants more power.
Eventually my Forte IIIs were replaced by JBL L100 Classics (I’m after a little vintage flare for this system) and shortly after that I went to (and still have) a Hegel H390 driving the JBLs. Before I sold the Fortes, I hooked them up in my main system which at the time was a McIntosh C53 and MC462. I heard what I think Klipsch intended for these speakers to be with the big amp. The MA8950 bears resemblance to the C53/MC462 and with a lot of power compared to the Line Magnetic. For this reason, and because you may find the 5-band EQ helpful, this could be a compelling pairing.
I don’t know Tekton well though I have read good things, especially for the money. If what I’ve read is accurate, they’re efficient, fast and dynamic and may tilt a bit forward. In the past, I’ve found nimble, energetic speakers to be a good fit with McIntosh.
It’s been a while since I’ve heard Maggies but my guess is that either Mac or Parasound would be a good fit there. Quite a stark contrast from the Forte or the Tekton, which I’m guessing is what peaked your interest. There’s a tweaky audio geek in me that wonders if you’d fall back in love with the Line Magnetic driving the Maggies. That is, as long as you’re not looking to rattle the walls...
Of note, the DA2 DAC module is the 8950 is pretty outstanding for what it is, but you’re paying for it. I’m not sure you’ll need it, having the Aurender though. And there’s the rest of the equation which is, the 8950 is a feature-laden integrated amp while the A21 isn’t.
Ultimately, both are good lines from good companies. McIntosh is legendary, has more prestige and great resale value. Parasound still works like a small business with an eye decidedly on value. They’re both great to work with.
As you’ll notice, everyone has their own experiences and opinions, and they can vary pretty widely. Bottom line is, you’ll only know what works for you once you’ve experienced it.