What 50wpc or higher amps have a totally different sound from Pass Labs, in a good way?

Keep what you have. 
 Not much better. If you want diff sound,..get new,speakers, keep the amps. Not much better. 
Speakers make the biggest difference!

Saw the recommendation for the Sumo Polaris... it is the most odd duck of the Sumo line and will sound different than Pass.  However Jim did not design this amp.  Rich May did it.  Jim would Never use mosfet outputs. The original Andromeda was a JB piece, but it could be edgy from the mids & up.  The Andromeda II was a similar flavor to the Polaris,  but everything is better on the Andromeda II than the Polaris.  It is odd that its a cool sounding amp, yet never gets sterile or harsh.  The Andromeda III is an update by Jason Stoddard.  The Polaris is a good recommendation non the less to hear a different sound than Pass.  

Search your local Craigslist for a used AV receiver for say $300 or  less. That should get you what you're looking for, the polar opposite of Pass Labs. 

The Xa 25 is designed differently. Most Pass amps are voiced with 2nd harmonic distortion. Older ones use different transistors as well. So there is that. MACS sound different with lots of power. Lots of fans.

I had Ampzilla and Quattre originals and remember power with good sound but i think similar to Pass.  

@wolfie - how do you think the thresholds differ sonically from the pass gear?