I have had all 3 amps that you’re considering here just recently. The Mesa was Triple Rev G. I went and listened to Septicflesh to get an idea of the tones you’re shooting for. I would say the UU or the Herbert would certainly get you there.
The MK3 Herbert is what I ended keeping. For me, and this is coming from playing in a band mix, it is the more versatile amp and sits in the mix almost perfect. It rivals my other guitar players MB Roadster. It has more mid content and more presence than the previous versions. A tighter low end as well. In contrast the UU was almost, dare I say, too much, in the mix! But that was me trying to get it to cover multiple genres. So, if you’re just dialing it in to specific tones. Either will get you there!
View attachment 316734