More watts or better power ?

Your amp plays loud enough so keep it.  Subs crossed at 80 Hz or so will take more load off the Xa25.  The speakers will sound better with less IM distortion.

Don Sachs and Supratech make excellent tube preamps if you want to try one.

Speaker isolatIon is another thing to consider if not done.

Power power power!!! Watts are the result of power and work in concert of each other. It is like horse power and torque one works great if you want to pull a stump out of the ground and the other if you are racing in a straight line. 

I wonder if given how good your system is now, a good power regenerator might not make sense. I got the same PS Audio email and as I recall one of their power regenerators has an offer on it. I personally bought a P5, their smallest unit, and noticed a blacker background and more articulate sound, which I attribute to lower noise. I’ve built some DIY Pass equipment and know his power supplies are great, but think there’s still an argument for clean power. Painless to try. You can just return it if you don’t like it.

I disagree with the generalization that more power is better. My Marantz 1060 at 30 wpc sounds better than most amps with 100, 200, 300 etc. It is all about the sound. It drives my outdoor speakers and they can obtain concert level sound with excellent fidelity.

As you are exploring the SVS option, take a look at the B&W PV1D. I just added a DB3D to my Magicos and the result is nothing short of spectacular.

The very best sounding amps I've heard are all low in output.  But, these amps are not appropriate for many speakers and in situations where high volume is a requirement and/or the room is extremely large.  So there is a minimum amount of power that is needed, and that minimum is situational.  

It is hard to say whether the 50 watts you have is sufficient.  All else being equal, more power will at least afford you more headroom for the brief peaks when playing very loudly.  That may or may not be a sufficient basis to upgrade to higher power.  But, often there is a sonic price to more power.  I once heard a comparison between a 50 watt Rowland amp and a very similar Rowland amp that was much more powerful.  I actually preferred the sound of the lower powered amp, and the speakers used in the comparison where supposedly a power/current hog.  I don't know why this was the case--it could be that using a larger number of output transistors adversely affects the sound, it could be that some transistor amps don't sound as good when they are just loafing at a small fraction of their capability--but this showed me that more is not necessarily better.

All of my current amps are low-powered tube units (6.5 watts/channel being the brawniest), but, I have appropriately efficient speakers and I prize good low-volume performance over ability to shake the room.