I have owned the Quicksilver Audio mini-mono amps for around 4 years now. I have own a few different amps in the past – Classe Audio P100, Assemblage (by Sonic Frontier) ST-40 tube amp, Plinius SA100mkIII, and I have listen a few others in my system. While the quicksilvers may not be the best amps that I have owned in the last 15 years, they have certainly given me the most musical enjoyment, when paired with the right speakers. And being able to use different types of tubes is another advantage.
Just a bit of about what I think about each amp: the Classe Audio is a warm sounding transistor amp, but not detailed enough, and not enough bass drive for my Genesis 8300 speakers (which I own at that time). The assemblage is a fun DIY project, but I am not very fond of its sound – does not sound like a valve amp at all. The Plinius is great, lots of power (which drove my Audio Physics Virgo very well), but it is too big and hot for me, and for some reason, still lacks some musicality that some tube amps can provide. The Quicksilver mini-mono, when paired with the Merlin TSM, (or Harbeth LS3/5a which I previously owned, or the Image 404 that my friend RC owns), gives a nice warm and balance sound, with reasonable good bass (both quantity and quality). And it is quite transparent as well. But remember, it is only a 25W amp, so don’t expect it to be as powerful as SA100 and DON’T use it to drive low impedance speakers!
Now back to tube rolling on Quicksilver. I have used 3 different sets of power tubes since I have owned the amps. No, I don’t just go out and buy 5 different sets and do a shootout like some people often do. I have stopped doing A/B comparison for awhile, as after all these years, I figured that only long-term listening will tell the truth (the only time that I do an A/B comparison is when I need to decide which component to buy in a shop or within a couple of days). I just buy a spare set of tubes, and pick the set that I like and use them first. I used the Tesla EL34 that came with the amps, then the Valve Art KT66, and now the JJ KT77.
The 3 sets of valves give different sonic characters. Below is summary of what I think about each type when used in my system.
1) Tesla EL34
The EL34 gives a slightly warmer sound compare to the KT66. Quite coherent from top to bottom. However, it is not very transparent, and not very extended top end. I prefer the EL34 over the KT66 when used with LS3/5a, but KT66 over EL34 with the Merlin TSM.
2) Valve Art KT66
More detailed sound than the EL34. A bit more transparent in the midrange, slightly more deep bass and highs. But I found some discontinuity over the frequency range (maybe in the upper bass/lower midrange?) that make the sound a bit lean. With the LS3/5a, the sound is too lean for me, but quite nice with the Merlin.
3) JJ KT77
This seems to be the best of both worlds. It looks very similar to the EL34, but slightly taller. Very coherent sound from top to bottom. It goes deeper and has slightly more powerful than the other two. This is very important for my system as it is still lacking a bit of bass. It has transparent midrange but still gives a slightly warmish sound. Soundstage is nice and gives a better contrast from front to back. Very musical. I would pick this over the other two.
I bought the KT77 from Hong Kong during my holiday, but they should be available from Freedom Music (Note: I have never buy anything from freedom music, but have heard good things about them and worth a try).
How about the small signal tubes (the mini-mono uses 12AX7)? I have tried a few different types – Telefunken, Mullard CV4004, Electro-harmonix etc. (Sorry, I am not an expert of tubes so I can’t tell the year of production and the exact version of each tube), but the change of sound is relatively subtle compare to the power tubes . I will definitely change the power tubes first before rolling the 12AX7.