Best Amp for Revel Ultima Salon 2's?

Go try a PASS Laps x600.5 Those speakers need the power. I have a X250 with my revel f52s and they are great match

It is very unlikely that different amps will sound significantly better than the XPA-1s. Tubes won't sound much different, at least they shouldn't, unless they are poorly designed (like having a high and uneven output impedance), defective, or simply lack the ability to properly drive the load the Salon 2 presents.

The XPA-1s use a SOTA fully balanced circuit topology, and they have more than enough power for the Salon 2. If you insist on an upgrade just for the point of upgrading, IMO the most important spec to look for is a super-low signal-to-noise ratio, like 90-95db below 1W of output. Krell, Classe, Levinson, ATI, and a slew of other companies make amps like this, but, frankly, I think you'd be wasting your money. If you insist on wasting it, I think Krells are the most over-engineered of the bunch.

Since you have a CP800, you might consider a powered subwoofer to smooth out the bass and allow you to place the Salon 2s for best imaging without regard for bass smoothness.