I have not used that exact audison, so my opinion is just that.
But you realize that a fully regulated amp like the 403a, strictly speaking, has ZERO headroom?
Headroom technically is the ability of an amplifier to deliver more than rated power for short periods of time.
Fully regulated amps in the car audio world trade off dynamic headroom for consistent power capability regardless of voltage fluctuations. Your 403a does 50x2 and 100x1 with any 12V supply between those limits (as voltage drops, it does draw more current). Many of the old PPI, the "slash" JL, and a few others fall into this category.
Loosely-regulated amplifiers have some amount of dynamic headroom (audisons do fall within this category as far as I know, and the DLS A5 does as well, as well as most amps for car audio historically), but they require external stiffening of the voltage supply to have extremely consistent performance. You mention this slightly when spec'ing the audison's operating voltage.
So I guess what I'm saying is, I love Morel, and I love xtant, but running Morels on a 403 is always going to have the problem you describe. I would expect it. I bet it sounds great at medium levels, but it strains at higher listening levels and the Morels lack a little slam and a little sparkle.
So I would consider three options to address this:
1) buy that xtant 3300 I wish I was buying on Classifieds and double your fully regulated power.
2) Get more efficient drivers instead of the Morels - Hertz Hi_Energy 165s, for example. Those speakers surprised me a lot.
3) Try another three-channel. I have used a DLS A5 on Morel Ovation 6 comps, it did sound pretty good, but it sounded better with a Zapco DC350.2 on them. The audison would probably be similar. I don't know a dedicated 3-channel that has 100WPC to the fronts, and that was when my Morels really opened up.
Or buy the audison LRx 4.1k I have on the classifieds, and run the 150x2 to the Morels, and bridge the 75x2 to your W1. That's actually not a terrible idea... but you probably have a 4 ohm sub. That amp only bridges to 4 ohms on the rear channels.
Zapco DC1000 would work that way. Even inn low-Z mode you would be hugely upgrading the power.