A big part of it is the mids scoop. With a Fender/Marshall/Vox tonestack you can dial in a "flat" tone with the mids all the way up and treble and bass all the way down, but no matter what the Hiwatt tonestack will always have a mids scoop. Elementary inherently has a little more gain than Underground Accelerator, but I wouldn't call it high gain. It does have a very pleasing overdrive characteristic though.
When I play an Underground Accelerator I typically enjoy keeping it clean, and the controls end up around Bass 12:00 Mids cranked Treble 12:00 Gain 10:00. It has a really nice fat presence, and this EQ setting keeps the mids flat while boosting the bass and treble a bit for a mild scoop that just feels great.
When I play an Elementary, I end up with all the knobs turned all the way up, and it sounds amazing. It's commanding, it's aggressive, it's punchy, but it never feels like "too much".