I owned an RKV for many moons before finally giving it up. Following that I purchased a Headmaster, and have been happy ever since.
The RKV definitely has some unique features going for it that would've allowed me to pick it out of a crowd of amps blindfolded. I think the most impressive thing about it is the bass. Its bass authority is simply awesome. Yes that means that its bass is more bumped up then the typical amp. But it doesn't mean that the bass is hip hop butt banging ghetto blaster style. It's a bass that has serious heavy weight and impact to it. No other amp to date I've heard has been able to achieve that same degree of authority, although I haven't heard that many high end amps.
The other feature is an enormous soundstage. Jump from just about any amp below the RKV's pricepoint to the RKV, and there's just no mistaking that you definitely get something back for what you paid. The soundstage width is just huge, and very noticeable.
It definitely keeps the distinctive tube flavor in the midrange and treble. There's no mistaking that you are definitely listening to a tube amp when listening to the RKV. Midrange has a certain lushness to it, and treble is, while very well extended, also silky smooth.
The only fault that kept me from keeping mine was background hiss. Otherwise I enjoyed the RKV in every other way, and really couldn't fault it for anything. But because of that hiss, it brought me to my next amp...the Sugden Headmaster.
The Headmaster has a very solid state-like sound to it...no mistaking here, blindfolded, that you are listening to a solid state amp. It has a flat, neutral sound. There really is no sound characteristic to describe the Headmaster. It's not bright, it's not warm, it's not bassy, it's not smooth or rough. Just flat. What your source gives in, the Headmaster puts right back out. Because of this it has a very clean, open sound to it that could be considered boring to some, if one likes to normally use amps as a form of equalization. The Headmaster just simply slips into your system and disappears.
This clean, flat sound allows the Headmaster to be probably the most transparent amp I've heard to date. Every little detail and nuance of a recording will be passed along from the source, given your source is capable of excellent detail extraction. It also allows for a drop dead quiet backdrop for gorgeous extensions. The bass goes deep as the recording allows, and likewise with the treble. How much of those extensions you hear thereafter is affected by your source, and choice of dynamic headphones. The midrange of this amp is very flat and even...and if there's anything one might worry about, it's this region. Voices come through starkly naked, but lacking any sort of emotional warmth. Such is the trait of neutral-like equipment unfortunately.
As for comparisons, both the RKV and Headmaster have excellent extension, and do go above and beyond lower priced amps in this area. However the RKV does suffer at dynamic impact in comparison to the Headmaster. Cymbals lack a certain brashness, drums sound slightly softened, violins and cellos lack a certain sharpness. The Headmaster's bass does extend deeper then the RKV's bass, although it sounds leaner in comparison to the RKV's bass, which has that amazing authoritive impact. i.e. the Headmaster lacks that challenging weight in the bass.
The Headmaster's soundstage is not as remarkably wide as the RKV's. However within the Headmaster's soundstage, it does have a better airiness around instruments.
Midrange is a toss up. Sometimes it's nice to have that warmth to a voice, but sometimes it's also nice to just to hear the stark nakedness of a voice. Similarily this applies to wooden bodied instruments as well. I don't believe these two items can possibly play nice with each other, so pick your poison here.
Transparency belongs to the Headmaster...heck it's what the Headmaster is all about. The Headmaster allows one to hear very deep into a recording, depths one may have never heard...again provided that the source and headphone is up to snuff.
In a blindfolded test I could definitely pick out the RKV. It has some very obvious traits that are pratically exclusive to it. On the other hand if you never told me we were testing headphone amps, I would probably never know the Headmaster even existed in a signal chain. And I think this is why I ultimately like the Sugden Headmaster. Drop one in, and it quietly goes about its job of amplifying your headphones while minimally messing around with the sound signal.
As a disclaimer, it goes without saying that if you're playing around with amps of this level, that the rest of your equipment should be up to snuff, particularly with the Headmaster.
I hope that helps a bit.
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