Douk Audio. Any experience with any of their products?

Hello. I have a DOUK AUDIO USB DAC. It replaces my previous Muse DAC.
In comparison, the Douk sounds much less congested and the music comes alive with lots of detail, a solid base and great imaging. 
I did have a problem with the driver that came with it.
I was getting TICS and TACKS at random while the music was playing but not on pause.
I tried everything under the sun to get rid of it but could not find the problem as I thought the problem was with my Dell D-630 when in fact it was with the Xmos drivers.
I did an update and now the problem is solved. 
This DACsounds great.

I bought their Dartzeel NHB-108 Clone. It's nothing short of amazing, by FAR the very best amplifier I've every owned ( and I've had Krell, Spectral, MBL etc, etc). The build quality is extraordinary. They also shipped super fast too. Couldn't be happier, in fact I like it so much I may well buy another one.

I found some images of the Dartzeel amp.  Looks amazing, but I don't think there is any connection with the audio kits and components sold on ebay under the Douk name.   

Any experence with their passive pre?

I got the "6j1 Valve tube pre-amp preamplifier bass on music fidelity X10-D"
the ad exactly. For $13 bucks. It was an easy kit to build, tiny but I have to admit it held up.
I first put it in between a emotiva DAC and threadhold stasis amp. First my Maggie’s, wasn’t impressed. Laughed cause $12 I’m glad it made sound. But.. it started to warm up and sure I can hear that tube sound. Hooked up a pair of speakers with ribbons. It continued to open up. 
My preamp is a audible illusions modulus 1 tube and This little plastic pretty blue light toy sounded pretty good. damn well worth the $13 I spent. Perhaps,
Improving the cheap components like caps and resistors would make a serious difference. Tubes of course would make a serious deference.
The little engine that could. It's a fun hobby. 

Any particular tubes work with this size tube? These are smaller them