Subtantial improvement over Emotiva XPA-1's?

My experience is that improvements come in small increments and sometimes the sound just takes on different qualities. I would advise you to be sensible about the improvements you expect or you might be disappointed. On the other hand,
unless there is a financial risk it is just fun to try new gear.

Leog2010, whether or not these amps bring out the best in your speakers is a roll of the dice. And it's a "dicey" game. You may have to just roll the dice and take the financial risk. All the brands you mention should on paper be a sonic step up from Emotiva; you will not know anything until you hook up the new amps and listen to your speakers, but you could end up having to re-sell. Not familiar with Theta, but Clayton and McIntosh have great reputations for quality. But good quality amps mated with the wrong speakers lead to dissatisfaction. Here's a thought, get some ideas for amps from the speaker maker Tyler.

Also, nypr2003 has XPA 1's and has owned Tyler speakers too. Might want to contact him.

I'd go with tubes and not worry about the D1's bass response. Supplement with a small sub/s that would have control your rooms bass EQ.

You'd end up with the lushness of tubes and deeper more controlled bass.