Considering the Parasound A23+ amplifier

A21 is the amp you want if your looking into that series. 
 Is the a21 a Curl designed amp, yes?

  

I highly doubt that the Pass XA25 is causing the lean sound with bright highs.  I suspect it is something else.  Maybe something in your preamp, or the type of 6SN7 tubes?  Or are you using silver wiring - as this will definitely slant the sound towards lean/bright if you have too much silver.

As far as Parasound A23+ or A21 goes, I don't think it will solve your problem.  It may make sound even leaner than the Pass Labs (the Pass XA25 is going to be biased much higher into Class A and providing a thicker sound).  I have heard the XA25 and it's definitely not "lean" sounding.

The Paraound amps have an incredibly clean midrange, but the high frequencies can come across a bit dry (in my system it lacked engaging sound quality).  You can fix with a fuse upgrade, but it can become even slower.  I'm not sure if this is what you want since it still will not be as "thick/full" sounding as the XA25.

If you want an amplifier that is full sounding but with softer and laid back highs, I would recommend a McIntosh amplifier.  But like I said, I think something else is going on in your system that is causing the lean/bright effect.

You should be able to pickup a 2-4 year old Parasound A21 for the same price or under the A23+ price new. If set on Parasound that’s what I would do. There are probably lots of other great amps new and used in that range as well. The A21 is definitely on the warmer side. I find it also very detailed and dynamic as well. Great voicing IMO. Is it neutral?? No idea but sounds great to me.