Lots of good points have been made in this thread regarding synergy, source components, tube types, etc., so I won’t repeat them.
As a happy owner of a DiaLogue One for over 6 years, and a heavy/extreme metal lover, I can relate to some of the points made in the original post, but one of the facts that needs to be mentioned (and seems to have been overlooked) is the quality of the recordings themselves. In short, most metal and related genre releases sound terrible. It really is that simple. Harsh, bass-heavy or bass-shy, flat, compressed, shrieking, etc, etc... I know, since I own at least 3000 genre CDs (anything from Burzum to Maiden to Emperor to Heaven Shall Burn to Satyricon to Anathema to My Dying Bride to Goblin to Blind Guardian to Opeth to In The Woods to DragonForce to Rhapsody to... you get the idea!). I LOVE this type of music, but the reality is that most of that stuff is not meant to be reproduced on a hi-end audio system, and the Prima Luna is actually quite revealing for a tube integrated. Garbage in, garbage out. You are better off playing this sort of thing in the car, frankly. I know I tend to enjoy it more there.
There ARE many decent recordings in the genre, however, and in those cases the amp will shine, but it is true that you may be better off with a different unit if your tastes are 90% metal. I do listen to a lot of classical and soundtracks as well, and I mostly love what I hear from the PrimaLuna. I did try out many different tubes, etc., but I found this to be an expensive exercise in nitpicking and settled on the tried-and-true Gold Lion KT-88 re-issue, and after trying a couple of Telefunkens, EH, etc, put the stock smaller tubes back. To my ears, this combination sounds right 90% of the time.
For the record, I drive a pair of Sonus faber Elipsas SE’s with the DiaLogue One, with no issues. Source is an Oppo 105 connected to a 2TB drive. Cables are all Nordost. Analog is humbler: a Pro-Ject RM 5.1 with a Dynavector 10X5. But I mostly listen to digital these days.