I have previously owned the Parasound Halo A21.
I have had only one brief experience in hearing the Fet Valve 550 and that was quite a while after buying my current amp, the Butler TDB 2250.
It's difficult to determine or compare the differences between the Halo A21 and the AVA amp, because of the different gear upstream, the different speakers and different rooms.
But I can tell you what little I did hear and why I sold my Halo A21.
The Halo A21 is powerful, dynamic and packs good impact/punch. It is relatively smooth in most frequencies and on good recordings. But I had problems with harsh, edgie high frequencies. Try as I might, I couldn't tame down the high frequencies with the A21; even on the better recordings. I tried different sources, pre amps and speakers; along with room treatments. The room treatments helped a little, but not enough to satisfy me.
So I sold the Halo A21 and bought the Butler TDB 2250. No more problems with annoying high frequencies. Only on the worst of the worst of recordings do I find some harshness and even then, it's much more tolerable than the A21. But take note that I also improved my Pre Amp and CD Players.
What I heard of the Fet Valve 550 was in many ways similar to the TDB 2250, but with a slightly different sound. Both are smooth, extremely musical and powerful. Both put the boogie in my booty.
I found the harmonics.....tunefullness and smoothness very similar. And to tell you the honest truth, I could never decide which amps I love best between the Butler TDB 2250 and the AVA Fet Valve 550.
Both simply present a wonderful, beautiful, musical presentation of music.....I mean
MUSIC that gets me on the emotional level.
So I decided to put the Butler TDB 2250 and TDB 5150 in the living room HT/2-channel system, and I'm currently saving for the Transcendence Eight Pre Amp and Fet Valve Ultra 550, for my home office. And since I work from home the AVA gear will get a major work out. I listen to music all day as I work

Don't get me wrong here, I think the Halo A21 is a nice amp. And I enjoyed much of what it did, but when it comes to comparing it to the sound/performance of these other two amps and what you get for the price; in my opinion, the Halo comes no where near the AVA Fet Valve Ultra 550 or the Butler TDB 2250.