New/Used Amp recommendations for about $3k-$4k?

I’m a bit confused, are you saying you want to get rid of the UPA-1’s after hearing the XPR-1’s? Or do you currently have the XPR-1 and looking for something else?
Also you didn’t mention what you are using as a pre amp but you may want to consider making an upgrade there first. Lack of detail and cleanliness as you put it could imply that your pre amp is stifling the performance. If the concern was more about bass & dynamics, then carry on with the power amp swap. Personally I’d first try a tube pre amp like the Schiit Freya to open things up before changing the amps.

Changing the power amp can also help with these things, but it may make less of a difference depending upon what you are using for the pre.

Well there you go... a monster AV Preamp at the heart of your system and you aren’t satisfied w/ music performance. Let me guess, guitars could sound a bit meatier, brass a little brassier, vocals don’t have that holographic 3d realism, performer in the room with you type of thing. Maybe a few panes of glass could be in the way? If only there was a solution. Enjoy the SS amp merry-go-round.