Hi... just joined here a minute ago. Thought I'd add my 2 cents, though maybe you have bought the amp by now.
I've had the HT-20 head for about a year, thru an Orange cab... It's a brilliant amp, and does exactly what it 'should', but imo is not great for metal... It has saturation and gain for days, but for me, it is very British and loose - 'Voxy' almost... I don't like amps that are too tight and trebly/fizzy either, but for me, this amp is not great for metal. I love the harmonic richness of the distortion tones, but no matter how I EQ or turn the ISF toward the American side, it just becomes too flubby and loose as I turn the gain up for metal... I can get the 'voice' of the tone I'm after EQ-wise, but the clipping style of the distortion is just too saggy and spongy for me. I would recommend the Series 1 combo for metal over the HT amps, at least where Blackstar is concerned... The HT distortion, for me, is better suited to classic rock, blues, and saturated, liquid leads... Just my view of course... I use the Blackstar HT-Distx for my metal, which is good... Maybe a little too far too the other side with it's extreme aggression.
I'm keeping mine, because I love it regardless of that, and I play other styles of music that it does really well. The clean channel and reverb are superb for one thing. At the minute, I'm looking at getting a EL34 Peavey XXX, a Valveking, or HOPEFULLY a Mesa Boogie .50 Caliber +. Between that and my Blackstar, I should have most of my bases covered, and can use an A/B box to switch them.