Which amp is better for my system

Are you sure? That's a pretty big shift from beautifully balanced excellence to run of the mill hifi technically hard to criticize even though there's no there there. Coming from Threshold maybe a nicely modded McCormack DNA?

I owned the Classe 15 and it was an outstanding amp! I doubt you would go wrong with it!

https://www.stereophile.com/solidpoweramps/1194classe/index.html

Perhaps the following may be worth a look.Cheers!
https://forum.audiogon.com/discussions/threshold-stasis-2-worth-a-rebuild

You can’t go wrong with a Classe 15, or DR15. They’re bulletproof.

I've had both amps and actually followed the Threshold with a Classe 25 (rather than the 15 you mention).  I used a Theta Miles run directly into the amp and speakers were original Martin Logan Aerius followed by Acoustat 2+2 with the blue medallion transformers.  I briefly had a pair of medium sized VMPS but have no idea as to the model.   The Threshold was my first "hi-end" amp and I thought it would be a nice upgrade from the Counterpoint SA-100 I'd been using.  It was a mixed bag at best and the Counterpoint was certainly hampered by older lousy stock tubes when I traded it in.  The Threshold had a nice smooth airy sound and threw a spacious soundstage but images were indistinct and the bass was lacking in impact.  Compared to an original Adcom 555 we had on hand, the Threshold was easier to listen to but seemed a little ill-defined and the bass was loose.  I was underwhelmed with the purchase.

I kept if for about a year and then had an opportunity to trade it in on the Classe 25.  The Classe was a better amp in most regards.  It was more detailed and better defined with regards to images and the bass was in another league though the Adcom 555 did best it in the slam department.  I felt the Classe had moved me significantly in the right direction but sometimes something seemed a bit "off" with the Classe.  Eventually I determined that the Classe just didn't do timing like other amps.  Most everything with a beat frequently sounded ever so slightly slow.  I listened to a lot of ambient so this wasn't an issue but I found myself quite dissatisfied when the music should have gotten up and boogied, which the Classe 25 just did not do.  

The shop I bought the Classe from had a Threshold SA3.9e and a Krell KSA 100S and I took both home and ended up hating the Classe afterward.  I bought the Krell which did EVERYTHING better.  I like the Threshold a little better but it just didn't have the power to run the Acoustats.

With regards to what I'm using now that would beat those amps mentioned....  that depends on the sound you like.  I like lots of detail with very well defined images and I detest bloat.  I prefer silver cables but balance them out with rhodium plugs.   Having said that, solid state amps I've used in the last few years that outclass the Threshold and the Classe would include the Cambridge Audio 840W and Xindak XA8800NME monoblocks.   And the current solid state amp I like is the humble Jaton 5140.  These go for so little on the used market but the detail is excellent, very well defined images, and the bass is shockingly tight and has impact that few amps can touch in these lower price ranges.  Plus, being 4 or 5 channels, you can biamp your speakers with them.  I've had great success with this amp on a pair of Meadowlark Hot Rod Herons and Triangle Luna Stratos.    Anyway, given what the Threshold and Classe 15 go for on the used market presently, I think you can do a lot better.  Still it's all a matter of synergy and if you like what you heard, take my comments with a grain of salt.  Good luck with your quest.