what do these terms mean??
Go by the definition of euphoric - it means it is very pleasing to listen to. Drug-like, even. Sometimes it's the recording itself being presented as such, sometimes it's the system or the system as a whole but owing a lot to one component in the chain. It's very difficult to describe, although Schiit's description of the Valhalla (check out the product blurb on their site) might be what they mean.
I kind of have a similar ting with what I like about treble, which I sometimes describe as "ethereal." It's not enough to say that the treble is well-extended, as a lot of headphones and speakers can extend past 20khz, but anyone who has heard opera, Sarah Brightman, Nightwish (Tarja), etc on Sonus Faber speakers for example will know what I mean by it, which is that female vocals sound a little ghostly. Not veiled, but very forward, very powerful, but it sounds like she's right there and not singing through a microphone (and yet sings over the rest of the band). Not all recordings will sound like it, of course, but those who do probably should (of course they won't sound like that live, given part of the sound is the speakers; but like I said, it's like they're singing without mics, and the treble reverberates around the room). Imagine a soprano or alto singing in a small room vs a stadium using mics and speakers.
Ex. More on that treble thing, if you have or can get your hands on a good reference system built around headphones that extend well in the treble like the HD600 or K701 (they don't sound different on my ASG-1 iem, the response dips after 10khz and nosedives past 12khz), listen to these two recordings of the same song. Or heck try it with what you have anyway; the difference in the vocal mics used might still be audible enough through the SR60.