I had an Anthem D1 before and found it to be thin sounding. Friends told me if I didn't like the sound of the D1 I wouldn't like the Anthem amps.
Not sure I agree that, since you didn't like the D1, you wouldn't like the A5 or P5. The Anthem Statement amps do not sound "thin" to me. Nor do they sound warm. To my ears, they sound neutral.
Would a Parasound Halo A51 be a justifiable upgrade for the money??
This is a question that only you can answer. To me, the answer is yes, particularly if you can find an A51 used.
Again, I have no experience with Wyred 4 Sound, so I can't advise you about that.