Others may have a different definition, but for me, headroom allows the audio to breathe and not compress or become congested when the volume is turned up. My main relevant experience is with tweaking the power supply rail of tube powered guitar amps, so it could be a bit different.
Thanks for the initial impression because I am wondering whether I should consider going up to the Ember or dropping down to the Sunrise or maybe a Starlight. I already have a Polaris, where I've tweaked the input attenuation module, and is sounding pretty good at the moment. However, I'm still keen for a tube amp without spending a heap of cash. I don't need a preamp out, so the Solstice would appear to be ideal if it sounds the same as the Ember. If it's not quite there sound wise, then maybe the Sunrise or Starlight might be better value for money.