PassLabs 350.8 versus Ayre MXR Twenty

Cary Audio SLP-05, Passlabs XA60.5 driving Goldenear Triton Ones.
Detail, spaciousness, and clean tight Bass to die for with the Triton's built-in 1600 watt sub amps and six bass drivers.

I agree with others that suggest to audition both amps in your system, but realistically I don't know how you arrange for that.

FWIW, I owned the Ayre MXR-20s and the Pass Labs X350.8 at the same time.  My speakers are Rockport Altairs.  They're not a difficult load, but they need an amp with ample current to correctly manage the 15 inch woofers. The X350.8 did a better job with this which led to better speaker coherence so there was less gaps at various frequencies.  I used a decibel meter and test tones from 10Hz to 50KKz (way beyond my audible range and likely my dog's as well).  The Ayres had measurable gaps (lower DB levels) in the lower frequency range 50Hz - 120Hz area.  I think this is why I got better slam and a better overall presentation with the X350.8.

Then there's the cost factor.  Even if the 2 amps were the same cost, I'd still probably choose the X350.8.  At the retail and pre-owned prices of each, it makes the X350.8 a bargain IMO.

I have since moved on to a Boulder 2060 which I'll never part with, but it is large and heavy and likely overkill for your speakers.

If you can't audition both, then my vote would be for the X350.8.  Hope this helps.  

@cycles2 you are probably the only person i know who has owned both amps at the same time.

As I said, I have heard the non Twenty MX R's.

How would you compare the Twenty MX R's to the X350.8 sonically driving your Rockport Altair's - other than the control factor?

I considered getting the Rockport Atria's at one stage. Super heavy - it took 2 guys to move it into position at the shop.

@mikey8811 As I mentioned above I consider the X350.8 to be a bargain. Don't believe all the tube-like characteristics that Ayre claim in their marketing material. It's just marketing speak.  IMO, if an amp doesn't get the bass foundation correct, then you're always looking for what the problem with your system may be.  Once I replaced the MXR-20s with the X350.8, the bass coherence improved which improves the overall presentation.  I used to own Pass Labs XA100.5 and I know the X350.8 is Class A/B and technically shouldn't be as good as the XA100.5, but I've had a few people tell me they believe the X350.8 is a 'giant killer' in the Pass lineup.  
The difference with the Boulder 2060 is that as you increase the volume, it just gets louder and there's no difference in sound characteristics. The X350.8 and the MXR-20s seem a bit strained as you increase the volume.  Probably as they switch over to Class B, which is easy to tell on the X350.8, and a mystery on the MXR-20s.
Remember, most of the weight of the MXR-20s is the aluminum chassis.  I don't claim to be an electrical engineer, but when you open the hood, it's easy to see why they don't generate lots of current - there's just no muscle underneath.