Sorry guys, I got lost in listening for hours yesterday and again this morning
I'm already having to come from memory comparing to the X150.5 because I'm not likely to try to switch back and forth. First off, the X150.5 weighs 62 lbs and the XA25 a much more manageable 45 lbs. I pulled out the still warm X150.5 out of the bottom shelf of my rack and put the still cold-from-being-in-UPS-warehouse-and-delivery-vehicle. But I know you weren't asking about the weight and temperature of the amps (although interestingly the XA25 seems to run slightly cooler that I remember the X150.5. With an infrared thermometer, the XA25 runs about 120 degrees as measured in the middle cooling fins).
On first listen , the word that came to mind was "glowing" but that doesn't quite explain...all the produced sound just seemed more vivid. I'm not sure if it was because there was a blacker background; I wasn't consciously aware of noise on the X150.5, but I've previously mentioned that due to noise/buzzing the X150.5 worked best with the Susvara and somewhat the LCD-4, but my other HPs were unusable with the X150.5, but the XA25 is quiet with all of my HPs. It made think that when you see a stage production with spotlights on a performers and the performers stand out that much more from the black background around them. From memory, the 150.5 seemed warmer more lush, but of course less distinct.
Upon more extended listening ("extended" being relative since I've only owned this for about 24 hours) and with the amp fully warmed up (I also left it on overnight)...ok, this is all going to sound like audio/new equipment cliche, but it's the best I can describe...the most striking thing is how natural this all sounds...for one, the timbre is intoxicating...every sounds like it's supposed to sound whether it's piano, woodwinds, massed strings, guitar, drumstick on a cowbell or most any other percussion instrument sounds...right. There's harmonic richness and density, but not overly so. I think some of this is due to greater detail and every part of a sounds being produced, the attach, sustain, decay just all being so clean. Hearing the microdynamics in vocal vibrato sometimes made me feel like I could see into the performers throat and see their tonsils. Reproduced audio is all an illusion and this illusion works for me...so many of the clues that betray reality...sibilance, compression, thinness/clinical/analytical sound were minimized compared with the X150.5.
After dinner last night, I asked my wife to listen with the Susvaras while I listened along using my HEK SE on my HPA-1. I try not to ever ask her to listen to "equipment" but I wanted to play the Pas de Deus part of The Nutcracker, performed by Mariinsky Theatre Orchestra conducted by Valery Gergiev, which for me is my favorite movement and this performance is so expressive. She calls this one the "sad" one and listening on this setup had her in tears. While listening, she would call out the instruments as the melody was handed off from oboe to clarinet or comment on the harps in the background. She then wanted to hear all of her favorite movements from The Nutcracker. It was as much of a validation as I would have expected from someone who wasn't into gear/equipment at all.
So you may have to excuse my effusive descriptions since I've been like a kid on Christmas (oh wait, it is Christmas day...). The XA25 was a significant upgrade in my playback system (XP-12, XA25, Susvara. DAC is currently Holo Spring KTE, but Santa is coming back for an encore on Monday with a Holo May KTE DAC...) and it's hard for me to imagine looking to upgrade this (ha, famous last words...). The X150.5 was my first attempt at using a speaker amp (i.e. the one I had on hand) and that certainly opened up my experience with my Susvara, but the XA25 was the component that I picked with the intent of using it to power headphones and I couldn't be happier with it.