Wilson Alexia 2 lacking some bass

I would add 2 Rel subwoofers like the S 510 or the S 812. I just added 2x S 500 with my Shindo Masseto and Montille amp and Fleetwood Deville SQ. Wish did it before, it is really a match made in heaven. Perfect bass , better coherence and live sounding.You won’t regret it, believe me, will take your system to another level.

I had a pair of Wilson Alexia’s and upgraded to the Alexia V.  Additionally, I have a pair of Rel S812’s next to each Alexia. I would check the distance from your back wall to the speaker and see if moving it around helps.  Are tour speakers tied in? That will also help with bass.  I added the Rel s812’s to ease the lower end on Alexia’s.  Having had Nagra equipment and demo’d before my final purchase I don’t think it could possible be the amps. Those are some of the finest products ever made.  Also, note that in Nagra uses Wilson speakers in many demos and at many of their shares so the components are a good match.  I think you need to demo a pre amp before doing anything else.  I’m sure your dealer would let you demo one to see if it fits in your system. 

Forgot to mention the obvious. Reflections act like tone controls. Excess refelctions will make a speaker sound bright. Tame the mid/treble reflections and often bass will emerge like a galleon at low tide.  Don't forget the ceiling, especially between listener and speaker.