One reason I think the X3 sounds better is the Wolfson DAC chip, to me its smoother and more dynamic than the PCM chip in the X5 and many of the newer DAPs.
While I don't have a Solstice to do any comparisons my Ember has never lacked transparency and is quite dynamic (depends on tubes used-Telefunken CCa, Siemens CCa and Amperex SQ/PQ) to me but it doesn't have as good soundstage and imaging as my BH Crack, headphone jack of UD-301 or old Sony ES HT receiver.
One of the best reasonable tube amps I've tried is the Elise but since its price ($1300, includes shipping) has gone up a lot over the past year I don't consider it as good a bargain as it price from when it was first introduced ($600 which included shipping IIRC) but they have made some improvements (I haven't heard the latest version) so maybe the price increase is justified. Others in this price range: WA2, Icon HP-8 mk2, MicroZOTL2 and Mainline.
Yeah, I've never though about the Ember lacking transparency or dynamics either. Like you say the tubes used will of course make a major difference here as well as the DAC you feed it from. I've only ever used my Ember connected to the DAC out on the Burson Conductor V2+, which is probably the most transparent DAC I've ever heard. My preferred tube with the HD700 is a Sylvania 6SN7 WGTbut I also enjoy it a lot with a Amperex 6dj8 orange globe. I can easily see the Ember (and any other tube equipped amplifier) perform a lot worse with subpar tubes or dac's. As always matching will also matter.....